The optical switch is capable of supervising the performance of optical switching in standby channels, and includes: a collimator unit; an optical splitter; a light-gathering unit; and a rotatable mirror. The optical switch further includes: a mirror angle controlling unit which controls a reflection face angle of the rotatable mirror for each wavelength to switch ON/OFF of the light beam coupling to the optical output port for each wavelength reflected, and determines an optical output port position outputting light beams of the reflected wavelengths; and a monitor unit, installed on a return path of a light beam, which monitors a light beam whose optical coupling is made OFF.