To provide a heat exchanger easy to be designed for multiple connection due to easy connection with other heat exchangers, and provide a heat pump apparatus.

Each of heat exchangers 21 ... has a double pipe structure using a liquid transfer passage R in which a forward travel path tube 2A is provided inside a backward travel path tube 2B and thus heat medium transferred from the outside of a fermentation tank 10 through the forward travel path tube 2A is returned through the backward travel path tube 2B to the outside of the fermentation tank 10. The heat exchangers 21 ... are arranged in parallel in accordance with the size of the fermentation tank 10. The heat exchangers 21 ... arranged in parallel are connected to each other in accordance with a thermal condition of each fermentation tank 10, such as deviated generation of fermentation heat in the fermentation tank 10, so that the liquid transfer passages R of the heat exchangers 21 ... are connected to each other in a connection form, such as in parallel, in series, in a parallel and series mixture connection element group, or in a series matrix element group 20A in which heat exchange groups of the preceding connection forms are arranged in matrix. Thus, suitable heat exchange characteristics are achieved in accordance with the fermentation tank 10.