【文章內(nèi)容簡介】 in the hot water W having a high temperature of 93℃ 97℃ . for a long time. In order to prevent the direct contact of the tank 4 with the hot water W, although a heat insulating jacket member 26 may be provided to surround the coffee tank 4 to form an air space therebetween, influence of the temperature of the hot water W upon the coffee C cannot be pletely avoided even though the jacket 26 is arranged. In addition, as the cock 1 is usually positioned outside the casing 50 in the air, temperature of a first cup of coffee after a nouse long period is low in parison with that of the coffee C in the tank 4 because at first heat of the coffee C is spent for warming the cock 1 and the temperature of the succeeding cup of coffee would be again about 93℃ 97℃ . Thus, cups of coffee having uniform and desirable temperature cannot be obtained through the cock 1. Moreover, it is impossible to adjust the temperature of prepared hot drink in the storage tank so as to be suitable for kinds of drinks. When a certain amount of hot water W has been spent on for brewing coffee and taking out of several cups of coffee and the water level in the tank 2 lowers, the level detector 11 operates to control the electromagic water supply valve 14 so as to supply water into a bottom portion of the tank 2 from a water source such as water service through a pipe 27 connected thereto. Since the supplied water is cold, it will stay at the bottom portion of the tank 2 and cool the coffee tank 4 if the bottom thereof is positioned at a relatively low position. In order to obviate this advantage, it is required to locate the coffee tank 4 at a relatively high level in the ta nk 4, which fact makes the volume of the tank 2 enlarge and requires much time to heat up the water in the tank to a predetermined temperature. Moreover, the manufacture of a large tank will bring about constructional problem such as welding work and require much time and expense for the manufacture. In order to obviate the defects of the automatic coffee urn of the type described above, is provided an automatic coffee urn in which a coffee storage tank is located in a casing independently of a hot water tank, for example, as described in . Pat. No. 3,149,556 to James S. Martin. In James, a coffee storage tank is located independently of a hot water tank and heated by a heater disposed only at the bottom of the storage tank. However, since the storage tank is heated only from the bottom, the whole wall structure of the storage tank is not uniformly heated and temperature difference exists at upper and lower portions thereof, ., the brewed coffee in the storage tank. Therefore, the coffee having a constant temperature cannot be obtained successively. SUMMARY OF THE INVENTION An object of this invention is to obviate defects of a prior art apparatus and provide a hot drink maker capable of obtaining cups of the hot drink successively having substantially the same temperature suitable for drinking. Another object of this invention is to provide a hot drink maker in which a hot water tank and a prepared hot drink tank are independently located and maintained so as to always keep predetermined temperature, respectively. According to this invention, there is provided a hot drink maker to be installed in a food service shop or restaurant prising, a casing provided with an opening at a front wall thereof, a hot water tank located in the casing and filled with hot water heated to a predetermined temperature, means for controlling water supply into the hot water tank, a funnel filled with material for preparing hot drink and detachably fitted in the opening, a spray head located above the funnel, a hot water supply pipe interconnecting the hot water tank and the spray head for supplying the hot water from the hot water tank into the funnel, a storage tank for storing prepared hot drink disposed in the casing just below the funnel, a heat insulating box arranged so as to surround The storage tank with space there between, a heater disposed in the space for heating uniformly the stock tank to be a predetermined temperature, and a cock for taking out the prepared hat drink in the storage tank through an outlet pipe extending from the bottom of the storage tank. BRIEF DESCRIPTION OF THE DRAWINGS In the acpanying drawing: FIG.. 1 is a schematic crosssectional view of a prior art automatic coffee urn。 and FIG. 2 is a schematic crosssectional view of an automatic coffee urn according to this invention. DESCRIPTION OF THE PREFERRED EIVIBODIMENT Referring to FIG. 2, an automatic coffee urn according to this invention prises a vertical casing 150 made of stainless steel, for example, and the casing 150 is provided at its front wall with an opening 125 in which a coffee funnel 105 is located. A cylindrical watertight tank 122 in which hot water W is contained is disposed in the casing 150 at a relatively high position and a heat insulating box 103 is disposed at the low portion in the casing 150. A coffee storage tank 104 for storing brewed coffee C is disposed in the heat insulating box 103 in a suspended manner such that an upper flanged