To solve problems on heat pump water heater which becomes widely used for a view point of CO2 reduction, that the heat pump water heater is not widely used in a collective housing as a hot water storage tank is large, and a coefficient of performance of the heat pump is small (heavy external work is required) as the hot water is stored at a temperature higher than a hot water supply temperature in usage, to reduce a dimension of the tank of which percentage of manufacturing costs is high.

A pressure and a temperature are optimized to be close to reversed Carnot Cycle on a T-S diagram, and simultaneously a cycle for introducing expansion turbine and recovering power is introduced. Further a compact flow channel structure of a microchannel radiator of high performance essential for achieving the cycle is constituted. The spread of the heat pump water heater including that to the collective housing is promoted by providing an instantaneous water heater having the characteristics.