In a valve timing adjusting apparatus, an outer peripheral surface of each projection of an internal gear, which is received in a corresponding engaging hole of a guide rotator, may have reduced surface sections, which are radially inwardly reduced toward a radial center of the projection along a radial line of the internal gear that passes through the radial center of the projection to have a reduced radial size in comparison to a radial size of the rest of the outer peripheral surface of the projection. Alternatively, an inner peripheral surface of each engaging hole may have recessed surface sections, which are radially recessed from the rest of the inner peripheral surface of the engaging hole along a radial line of the guide rotator that passes through a center of the inner peripheral surface of the engaging hole.