An object is to carry out an abnormality diagnosis for a variable compression ratio mechanism without adding any special hardware configuration for estimating a compression ratio. An abnormality diagnostic device for a variable compression ratio mechanism adapted to be applied to an internal combustion engine comprises a controller configured to calculate a predetermined rotational speed difference which is a rotational speed difference, between an engine rotational speed at a predetermined crank angle before or after compression top dead center and an engine rotational speed in the vicinity of the compression top dead center for a predetermined cylinder at the time of carrying out fuel cut processing, set a reference range of the predetermined rotational speed difference, and make a diagnosis that abnormality has occurred in the variable compression ratio mechanism when the predetermined rotational speed difference is out of the reference range.