A heat engine includes: a high-temperature space portion and a low-temperature space portion, each of which has a working gas with a different temperature range from each other; a regenerator provided between both of the space portions; a first piston configured to cause volumetric changes of the working gases in the space portions and transmit motive energy on receipt of pressure changes of the working gases; and a second piston and a third piston configured to transfer the working gases between both of the space portions and move with a 180° phase difference from each other with respect to the regenerator. The second piston is slidably housed in a cylinder portion included in the first piston. Heat and motive energy are exchanged by using the volumetric changes in both of the space portions, as well as by using the transfer of the working gases.