It is an object of the present invention to provide a heat pump that can use, as a refrigerant, a substance with which a geometric isomer exists, and a method for designing the same. The present invention is a method for designing a heat pump (1) whose closed circuit configured by connecting a compressor (2), a condenser (3), an expansion valve (4), and an evaporator (5) in this order is filled with a refrigerant containing a refrigerant substance with which a geometric isomer may exist, the method including obtaining an upper limit of stable temperature at which isomerization of the refrigerant substance does not proceed, and setting an upper limit usage temperature of the heat pump (1) so as not to exceed the upper limit of the stable temperature.