A hot water supply apparatus comprising: a hot water tank, heating means which heats water in the hot water tank, a first pipe connected to a lower portion of the hot water tank and arranged to send the water to the heating means, a second pipe connected to an upper portion of the hot water tank and arranged to return the water heated by the heating means to the upper portion of the hot water tank, a hot water supply pipe connected to the upper portion of the hot water tank and arranged to take out the hot water from the upper portion of the hot water tank, and a water supply pipe connected to the lower portion of the hot water tank and arranged for enabling water to flow into the lower portion of the hot water tank; a heat exchanger which is formed with a pipe for the heat exchanger in the hot water tank and arranged for increasing the temperature of the water flowing through the pipe for the heat exchanger by exchanging heat between hot water in the hot water tank and water flowing through the pipe for the heat exchanger wherein the pipe for the heat exchanger is formed spirally as a single path; wherein an outer end of the pipe for the heat exchanger disposed in the form of a mortar in the heat exchanger is connected to a first connecting tool, and an inner end of the pipe for the heat exchanger is connected to a second connecting tool, a bathtub, a third pipe arranged for sending water in the bathtub to the heat exchanger, a fourth pipe arranged for returning the hot water after it is heat-exchanged from the heat exchanger to the bathtub, the first connecting tool arranged for connecting the heat exchanger to the third pipe, and the second connecting tool arranged for connecting the heat exchanger to the fourth pipe, characterized in that the upper portion of the hot water tank is formed substantially in a semicircle spherical shape, a heat pump cycle which is operated with a pressure over a critical pressure is used as the heating means, wherein, in use, hot water accumulated in the upper portion of the hot water tank is taken out from the hot water supply pipe and is used as hot water supply, and an amount of water corresponding to the hot water taken out from the hot water supply pipe is filled into the lower portion of the hot water tank from the water supply pipe, and the pipe for the heat exchanger is disposed in the upper portion of the hot water tank wherein adjacent lines of the pipe are located such that an inner line is higher than an outer line in the form of a mortar whose opening area is increased downwardly, and wherein, in use, water is provided from the bathtub to the heat exchanger through the third pipe and the water is provided from the outer line into the inner line and the water is returned to the bathtub through the fourth pipe. a hot water tank, heating means which heats water in the hot water tank, a first pipe connected to a lower portion of the hot water tank and arranged to send the water to the heating means, a second pipe connected to an upper portion of the hot water tank and arranged to return the water heated by the heating means to the upper portion of the hot water tank, a hot water supply pipe connected to the upper portion of the hot water tank and arranged to take out the hot water from the upper portion of the hot water tank, and a water supply pipe connected to the lower portion of the hot water tank and arranged for enabling water to flow into the lower portion of the hot water tank; a heat exchanger which is formed with a pipe for the heat exchanger in the hot water tank and arranged for increasing the temperature of the water flowing through the pipe for the heat exchanger by exchanging heat between hot water in the hot water tank and water flowing through the pipe for the heat exchanger wherein the pipe for the heat exchanger is formed spirally as a single path; wherein an outer end of the pipe for the heat exchanger disposed in the form of a mortar in the heat exchanger is connected to a first connecting tool, and an inner end of the pipe for the heat exchanger is connected to a second connecting tool, a bathtub, a third pipe arranged for sending water in the bathtub to the heat exchanger, a fourth pipe arranged for returning the hot water after it is heat-exchanged from the heat exchanger to the bathtub, the first connecting tool arranged for connecting the heat exchanger to the third pipe, and the second connecting tool arranged for connecting the heat exchanger to the fourth pipe, characterized in that the upper portion of the hot water tank is formed substantially in a semicircle spherical shape, a heat pump cycle which is operated with a pressure over a critical pressure is used as the heating means, wherein, in use, hot water accumulated in the upper portion of the hot water tank is taken out from the hot water supply pipe and is used as hot water supply, and an amount of water corresponding to the hot water taken out from the hot water supply pipe is filled into the lower portion of the hot water tank from the water supply pipe, and the pipe for the heat exchanger is disposed in the upper portion of the hot water tank wherein adjacent lines of the pipe are located such that an inner line is higher than an outer line in the form of a mortar whose opening area is increased downwardly, and wherein, in use, water is provided from the bathtub to the heat exchanger through the third pipe and the water is provided from the outer line into the inner line and the water is returned to the bathtub through the fourth pipe.