To improve output in a series double-acting type four cylinder hot gas engine by connecting one end of a heater consisting of a heater tube to a cylinder directly and connecting the other end thereof to a regenerator directly to make the total length and a volume of the heater tube equal.

Concerning a first heater tube group 37, one end thereof 37a is directly connected with a first cylinder 1, and the other end thereof 37b is directly connected with a first regenerator 21. In the same way, with respect to a second heater tube group 38, one end thereof 38a is directly connected with a second cylinder 2, and the other end thereof 38b is directly connected with a second regenerator 22. Accordingly, it is unnecessary to provide connection duct parts which connect each heater tube group 37, 38 with each cylinder 1, 2 and each heater tube group 37, 38 with each regenerator 21, 22 therebetween, respectively, thereby reducing dead volume by an amount by which the connection duct parts are eliminated. For this reason, it is possible to decrease a volume of a part which is hardly related to heat exchange such as the connection duct.