Actuator of link mechanism for internal combustion engine

An actuator of a variable compression ratio mechanism of an internal combustion engine, comprising:

• a control shaft configured to rotate to vary characteristics of the variable compression ratio mechanism of the internal combustion engine;
• a speed reducer configured to reduce and transmit a rotation rate of an electric motor to the control shaft;
• an oil passage formed inside the control shaft;
• and
• an open portion which opens to face the speed reducer at one end side located on a side of the speed reducer, and which is in communication with the oil passage at another end side of the open portion,
• wherein an inner diameter of the one end side of the open portion is larger than an outer diameter of the control shaft,
• wherein the control shaft includes a flange on one end side, which is connected to the speed reducer, and
• wherein a length of the open portion from the one end side of the open portion to the another end side of the open portion in a rotation axis direction of the control shaft is greater than a thickness of the flange.