To provide a control device for a variable compression ratio internal combustion engine capable of controlling an engine based on a value close as possible to a true mechanical compression ratio even when cylinder internal pressure detected by a cylinder internal pressure sensor is remarkably different from a true cylinder internal pressure.

This control device is applied to an internal combustion engine 10 equipped with a mechanical compression ratio changing mechanism. This control device obtains a mechanical compression ratio as an actual mechanical compression ratio based on the cylinder internal pressure detected by the cylinder internal pressure sensor 45. The control device transmits a command signal for changing the mechanical compression ratio to an actuator 15M of the mechanical compression ration changing mechanism. The control device obtains the mechanical compression ratio based on the command signal as an estimated mechanical compression ratio. The control device adopts "a prescribed mechanical compression ratio between the actual mechanical compression ratio and the estimated mechanical compression ratio" as the mechanical compression ratio used to control the engine 10 when "a difference between the actual mechanical compression ratio and the estimated mechanical compression ratio" is larger than a prescribed threshold value.