A valve timing adjusting device includes: a first rotor rotating integrally with a crankshaft; a second rotor integrally secured to an intake or exhaust camshaft; and an engaging cavity provided in one of the first rotor and the second rotor. A lock pin is housed in a housing hole provided in the other of the first rotor and the second rotor, protruded by the energizing force of an energizing device at the time of hydraulic pressure reduction, to thus abut on the wall of the engaging cavity from an oblique direction to give a relative rotating force to the first rotor and the second rotor.