A valve timing adjustment device for promptly switching a rotational phase includes
a guide rotary body having a guide passage containing a movable body. The guide
passage includes a variable radial dimension relative to a rotational centerline.
The guide rotary body rotates relative to a driving rotary body and guides the
movable body in the guide passage. A phase change mechanism changes the rotational
phase of a driven shaft relative to a drive shaft according to the position of
the movable body in the guide passage. The guide passage has a gradually decreasing
region and a gradually increasing region. The gradually decreasing region slants
toward an axis of the guide rotary body. The gradually increasing region slants
relative toward the axis of the guide rotary body. The gradually decreasing region
and the gradually increasing region are connected.
