A valve timing control system has a tubular housing; a cam shaft having an external
periphery formed with a drive cam; a phase variation mechanism disposed in the
tubular housing, and varying a rotational phase of the sprocket portion relative
to the cam shaft in accordance with oil pressure supplied to the phase variation
mechanism; and an oil pressure control measures for controlling the oil pressure
supplied to the phase variation mechanism. The tubular housing has a housing body
having a density, and a sprocket portion for receiving a drive force transmitted
from a crank shaft of an engine by way of a chain. The sprocket portion is disposed
integrally to the tubular housing, and has a density higher than the density of
the housing body. The tubular housing is so mounted to the cam shaft as to make
a rotation relative to the cam shaft when so required.
