A variable compression ratio internal combustion engine is provided with a variable compression ratio mechanism in which a mechanical compression ratio of the internal combustion engine changes in accordance with a rotational position of a control shaft, a low compression ratio side stopper, a high compression ratio side stopper, a sensor for detecting a rotational position of a drive shaft of an actuator, and an arm press-fitted onto the drive shaft. Relative rotation between the arm and the drive shaft occurs when a torque exceeding an upper-limit torque has been applied. The drive shaft or the control shaft is caused to move to restriction positions restricted by the respective stoppers, and then a diagnosis on the presence or absence of the relative rotation is executed, based on the detected values at the respective restriction positions.