To prevent or suppress formation of level difference in the inner surface of a cylinder bore.

A pair of pistons 33A, 34A (33B, 34B) arranged in series to each other are slidably fitted in a cylinder housing 31, and a combustion chamber 35A (35B) is formed between the pair of pistons. This free-piston engine includes a position changing means which changes the relative positions of the pair of pistons 33A, 34A (33B, 34B) in an axis direction with respect to the cylinder housing 31 while constantly maintaining a distance between the pair of pistons 33A, 34A (33B, 34B). The relative positions thereof are changed in a manner that the position of a piston holding housing 41 slidably holding the pair of pistons is changed with respect to the cylinder housing 31 by a motor 71, or a power generation mode between the pair of pistons is changed.