An actuator apparatus for an internal combustion engine including a variable compression ratio mechanism, wherein the variable compression ratio mechanism includes a control shaft, and is configured to vary a mechanical compression ratio depending on rotational position of the control shaft, the actuator apparatus comprising: an actuator configured to rotate the control shaft; an auxiliary shaft whose rotational position is controlled by the actuator; a link mechanism configured to link the control shaft with the auxiliary shaft, wherein the control shaft is disposed inside of an engine body (3, 4), and wherein the link mechanism includes: a first arm configured to be attached to the control shaft; a second arm attached to the auxiliary shaft; and an intermediate link configured to link the first arm with the second arm; a housing configured to be attached to a side wall of the engine body (3, 4), and configured to support the actuator and accommodate the auxiliary shaft; an oil filter mount part formed as a part of the housing; and a stopper part including a stopper surface configured to be in contact with the second arm so as to define a first bound of rotational position of the auxiliary shaft; wherein the housing includes first and second wall parts (45, 46) perpendicular to the auxiliary shaft, wherein the two wall parts (45, 46) define a space in which the second arm is configured to swing; wherein the housing includes an outside wall part configured to connect the two wall parts (45, 46) to each other; and wherein the oil filter mount part is continuous with a lower end of the outside wall part and is formed integrally with the stopper part; characterized in that the stopper part is configured to connect the first wall part to the second wall part. an actuator configured to rotate the control shaft; an auxiliary shaft whose rotational position is controlled by the actuator; a link mechanism configured to link the control shaft with the auxiliary shaft, wherein the control shaft is disposed inside of an engine body (3, 4), and wherein the link mechanism includes: a first arm configured to be attached to the control shaft; a second arm attached to the auxiliary shaft; and an intermediate link configured to link the first arm with the second arm; a first arm configured to be attached to the control shaft; a second arm attached to the auxiliary shaft; and an intermediate link configured to link the first arm with the second arm; a housing configured to be attached to a side wall of the engine body (3, 4), and configured to support the actuator and accommodate the auxiliary shaft; an oil filter mount part formed as a part of the housing; and a stopper part including a stopper surface configured to be in contact with the second arm so as to define a first bound of rotational position of the auxiliary shaft; wherein the housing includes first and second wall parts (45, 46) perpendicular to the auxiliary shaft, wherein the two wall parts (45, 46) define a space in which the second arm is configured to swing; wherein the housing includes an outside wall part configured to connect the two wall parts (45, 46) to each other; and wherein the oil filter mount part is continuous with a lower end of the outside wall part and is formed integrally with the stopper part; characterized in that the stopper part is configured to connect the first wall part to the second wall part.