Disclosed is a variable compression ration mechanism (5) in which a control shaft (10) is rotated and driven by an electric motor (21) through a wave gear reducer (22). The wave gear reducer (22) includes: a first internal gear member (31); an external gear member (32) arranged concentrically inside the internal gear member (31); a wave generator (33) arranged inside the external gear member (32); and a second internal gear member (34), wherein a ratio of the number of teeth of a fixed gear portion (35) of the first internal gear member (31) to the number of teeth of a first gear portion (37) of the external gear member (32) is set lower than a ratio of the number of teeth of an output-side gear portion (42) of the second internal gear member (34) to the number of teeth of a second gear portion (38) of the external gear member (32). Thus, the tooth depth of the output-side gear portion (42) of the second internal gear member (34) is made relatively large so as to attain a higher ratcheting torque and prevent ratcheting.