An abnormality diagnosis system 100 includes a VVT controller 110, an adjustable valve mechanism 120, a cam angle sensor 130, a model computation module 140, an abnormality detection module 150, and an alarm lamp 160. The adjusable valve mechanism 120 advances or delays a phase angle of an intake cam shaft relative to a crankshaft, thereby varying an open-close timing of a valve. The model computation module 140 computes a physical behavior of the adjustable valve mechanism 120 according to a physical model, in response to a control signal input from the VVT controller 110. The abnormality detection module 150 determines whether the adjustable valve mechanism 120 is abnormal or normal, based on a difference between a theoretical value of phase angle calculated according to the physical model and an observed value of phase angle measured with the cam angle sensor 130. The arrangement of the invention ensures highly accurate abnormality diagnosis of the adjustable valve mechanism 120.