In a valve timing adjusting apparatus, rotating response speed of a vane rotor and a moving speed of a lock piston are changed according to changes of oil temperature and pressure. It sometimes happens that the lock piston passes the fitting hole before the lock piston is fitted in the fitting hole. Timing of actuating a solenoid valve is retard by a given delay time from timing of actuating a spool valve. The given delay time is decided by a map based on sensor signals representing the oil temperature and pressure input to ECU. The given delay time is shorter as the oil temperature increases and longer as the oil pressure increases.