An abnormality diagnosis apparatus for an internal combustion engine in which a phase of a first valve provided to a first cylinder is modified within a range extending from a first phase to a second phase and a phase of a second valve provided to a second cylinder is modified within the range includes: a rotation variation amount detection unit that detects a rotation variation amount of an output shaft of the internal combustion engine; and a diagnosis unit that adjusts the phase of the first valve and the phase of the second valve to the first phase, and diagnoses the presence of an abnormality on the basis of the detected rotation variation amount of the output shaft of the internal combustion engine in a state where the phase of the first valve and the phase of the second valve are at the first phase.