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 compression ratio detection unit that detects an actual compression ratio. In a predetermined engine operating condition (S11) such as immediately after an engine start, reference position learning operation for the compression ratio detection unit is carried out (S12) 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 due to a communication error or the like (S13), or even when a difference between the actual compression ratio and a target compression ratio becomes greater than an assumed deviation due to some abnormalities (S14), the reference position learning operation is immediately carried out (S15), thereby enabling a quick return to a state where normal compression ratio control can be carried out.