A camshaft is formed at a generally middle portion with an enlarged cylindrical portion. An annular vane member is mounted on the enlarged cylindrical portion and secured thereto by means of a nut. An annular housing or sprocket is arranged to receive therein the annular vane member in a manner to permit a rotation of the vane member relative to the annular housing. Each of a circular opening of the vane member and a circular opening of the annular housing is so sized as to permit cam lobes of the camshaft to pass therethrough.