Provided is a device and method for detecting an optical signal. The optical signal detecting device may include an optical de-multiplexer configured to de-multiplex an input optical signal to optical signals of different wavelengths; an optical coupling lens configured to allow the optical signals of different wavelengths to be incident; an optical signal reflector configured to reflect the optical signals of different wavelengths emitted from the optical couple lens; and an optical detector configured to detect the reflected optical signals of different wavelengths.