The control shaft (20) of a variable compression ratio mechanism is coordinated with the auxiliary shaft (25) of an actuator device (1) through an intermediate link (31). The housing (5) of the actuator device (1) has on the lower surface thereof an oil filter mounting seat section (36), and a stopper section (37) is formed integrally with the oil filter mounting seat section (36). A second arm (28) of the auxiliary shaft (25) comes in contact with the stopper section (37) to define the limit of the compression ratio on the low compression ratio side. The stopper section (37) has high rigidity because the oil filter mounting seat section (36) having high rigidity and the stopper section (37) are integrated together.