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.