A control device for a compression ratio variable internal combustion engine is provided with a compression ratio variable mechanism for changing an engine compression ratio in accordance with a rotational position of a control shaft, and a detection unit that detects an actual compression ratio. In a predetermined engine operating condition such as immediately after an engine start, reference position learning operation for the detection unit is carried out with the control shaft mechanically locked up at a predetermined reference position. During operation of the engine, even when the actual compression ratio is lost or a difference between the actual compression ratio and a target compression ratio becomes greater than an assumed deviation, due to some abnormalities, the reference position learning operation is immediately carried out, thereby enabling a quick return to a state where normal compression ratio control can be carried out.