PURPOSE: To diagnose troubles, including mechanical, troubles in addition to electric troubles in a variable valve timing control device.

CONSTITUTION: An average effective pressure Pi is calculated based on a detected signal of a cylinder-internal pressure sensor 10. In this case, when the average effective pressure falls within a specified diagnosis range containing ON-range and OFF-range of variable valve timing control, an average effective pressure PiON in ON-controlled state and an average effective pressure PiOFF in OFF-controlled state are respectively determined. Further the deviation ΔPi (←PiON-PiOFF) of the average effective pressure PiON from the average effective pressure PiOFF is calculated, and when ΔPi is below a specified judging value, the variable valve timing control device is judged to be in trouble.