In a hydraulic variable valve timing mechanism including a lock pin that locks a vane rotor and a housing against relative rotation in the most retarding phase, a specified angle α is set so that release of the lock pin is started at a time when positive cam torque acts on the vane rotor. When the crank angle reaches the specified angle α, supply of hydraulic pressure to the advancing oil chamber is started to release the lock pin from the engagement with the lock hole, thereby enabling a reliable release of the lock pin prior to start of change of valve timing.