A driving-side rotatable body includes a driving-side inner gear, which
has an axial extent that does not overlap with an axial extent of a
driven-side inner gear of a driven-side rotatable body. A driven-side
outer gear and a driving-side outer gear of a planet gear are meshed with
and are driven together with the driven-side inner gear and the
driving-side inner gear, so that the planet gear changes a relative
rotational phase between the driven-side rotatable body and the
driving-side rotatable body. An addendum circle of the driving-side inner
gear has a diameter, which is larger than that of a dedendum circle of
the driven-side inner gear.