Disclosed is a rotation amount detecting system for detecting a rotation amount of a deflection mirror, which is rotatable about an rotation axis, employed in an optical disc drive. The rotation amount detecting system is provided with a light emitting system that emits a light beam to the deflection mirror. The light beam has a line-like cross section extending in a direction perpendicular to the rotation axis. Further, a detecting system is provided to receive the beam reflected by the deflection mirror and determines the rotation amount of the deflection mirror in accordance with the received beam.