To provide a compression ratio varying device of an internal combustion engine capable of securely switching a compression ratio without rotating a piston outer part.

This compression ratio varying device comprises a piston inner part 5a connected to a connecting rod 7, the piston outer part 5b fitted to the outer periphery of the piston inner part 5a slidably only in the axial direction, a restricting means 18 fixedly installed a head 5bh so as to axially oppose it with the piston inner part 5a, between a first cam mechanism 151 interposed between the piston inner part 5a and the head 5bh and controlling a distance S1 in a first axial direction between the piston inner part 5a and the head 5bh, and a second cam mechanism 152 interposed between the piston inner part 5a and the restricting means 18 and controlling a distance S2 between the piston inner part 5a and the restricting means 18.