A variable compression ratio mechanism for an internal combustion engine, comprising a gear reducer (22; 51; 61) that reduces and transmits rotation of an actuator as a drive torque to a control shaft so as to change a rotational position of the control shaft and thereby vary an engine compression ratio of the internal combustion engine, wherein the gear reducer (22; 51; 61) comprises: an input-side member (33; 55; 65) connected to a rotation shaft of the actuator; an output-side member (34; 54; 64) that transmits the reduced rotation of the actuator to the control shaft; a fixed member (31; 52; 62) fixed to a housing of the actuator; and an intermediate member (32; 53; 63) that transmits torque from the input-side member (33; 55; 65) to the output-side member (34; 54; 64) and the fixed member (31; 52; 62), wherein the variable compression ratio mechanism further comprises an output shaft to which the output-side member (34; 54; 64) is fixed such that the drive torque of the actuator is transmitted to the control shaft through the output shaft, characterized in that the intermediate member (32; 53; 63) has a first gear portion (37; 57; 67) brought into engagement with a fixed gear portion (35; 58; 69) of the fixed member (31; 52; 62) and a second gear portion (38; 57; 68) brought into engagement with an output-side gear portion (42; 59; 70) of the output-side member (34; 54; 64), wherein a ratio of the number of teeth of the fixed gear portion (35; 58; 69) to the number of teeth of the first gear portion (37; 57; 67) is set lower than a ratio of the number of teeth of the output-side gear portion (42; 59; 70) to the number of teeth of the second gear portion (38; 57; 68), wherein a rigidity of a flange part of the output shaft to which the output-side member (34; 54; 64) is fixed is higher than a rigidity of the housing to which the fixed member (31; 52; 62) is fixed, and/or wherein a thermal expansion coefficient of the output shaft to which the output-side member (34; 54; 64) is fixed is lower than a thermal expansion coefficient of the housing to which the fixed member (31; 52; 62) is fixed, and/or wherein the number of bolts by which the output-side member (34; 54; 64) is fixed to the output shaft is larger than the number of bolts by which the fixed member (31; 52; 62) is fixed to the housing. an input-side member (33; 55; 65) connected to a rotation shaft of the actuator; an output-side member (34; 54; 64) that transmits the reduced rotation of the actuator to the control shaft; a fixed member (31; 52; 62) fixed to a housing of the actuator; and an intermediate member (32; 53; 63) that transmits torque from the input-side member (33; 55; 65) to the output-side member (34; 54; 64) and the fixed member (31; 52; 62), wherein the variable compression ratio mechanism further comprises an output shaft to which the output-side member (34; 54; 64) is fixed such that the drive torque of the actuator is transmitted to the control shaft through the output shaft, characterized in that the intermediate member (32; 53; 63) has a first gear portion (37; 57; 67) brought into engagement with a fixed gear portion (35; 58; 69) of the fixed member (31; 52; 62) and a second gear portion (38; 57; 68) brought into engagement with an output-side gear portion (42; 59; 70) of the output-side member (34; 54; 64), wherein a ratio of the number of teeth of the fixed gear portion (35; 58; 69) to the number of teeth of the first gear portion (37; 57; 67) is set lower than a ratio of the number of teeth of the output-side gear portion (42; 59; 70) to the number of teeth of the second gear portion (38; 57; 68), wherein a rigidity of a flange part of the output shaft to which the output-side member (34; 54; 64) is fixed is higher than a rigidity of the housing to which the fixed member (31; 52; 62) is fixed, and/or wherein a thermal expansion coefficient of the output shaft to which the output-side member (34; 54; 64) is fixed is lower than a thermal expansion coefficient of the housing to which the fixed member (31; 52; 62) is fixed, and/or wherein the number of bolts by which the output-side member (34; 54; 64) is fixed to the output shaft is larger than the number of bolts by which the fixed member (31; 52; 62) is fixed to the housing.