A light-receiving module of the present invention includes: a lens collimating signal light entering from an optical fiber; a PLC (Planar Lightwave Circuit) where the signal light emitted by the lens enters; a mirror having a function of reflecting and transmitting the signal light emitted by the PLC; and a light-receiving element receiving the signal light reflected by the mirror. The mirror also has a function of transmitting light from the opposite direction to a direction to transmit the signal light emitted by the PLC. The module further includes a dummy mirror which reflects external light entering from outside in a direction toward the PLC, passes the reflected external light through the mirror from the opposite direction to the signal light transmission direction, causes the light to enter the PLC, and guides the light to the optical fiber via the PLC.