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 , 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 , 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 perpendicular to the auxiliary shaft , wherein the two wall parts 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 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 , 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 , 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 perpendicular to the auxiliary shaft , wherein the two wall parts 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 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 . <EOS>