A valve timing adjusting device includes a first rotor that rotates synchronously
with a crankshaft of an engine, a second rotor that is relatively rotatable by
a predetermined angle within the first rotor and is secured on the end face of
an intake or exhaust camshaft, and a lock mechanism that restricts a relative rotation
between the first rotor and the second rotor. The receiving hole of the lock mechanism
is a stepped structure having a small part provided at the outermost located radially
to the device.