To provide an optical signal processor which can perform favorable optical signal processing even when there are environmental changes and the like. An optical signal processor 1 inputs light emitted from an end face of an optical fiber 11, subjects the inputted light to processing according to its wavelength, and outputs the processed light so as to make it incident on the end face of the optical fiber 11; and comprises optical systems 111 to 113, a diffraction grating device 120, reflecting mirrors 131 to 133, an optical path turning part 140, and a monitor part 150. The optical path turning part 140 transmits therethrough a part of the incident light and reflects at least a part of the remnant. The optical system 113 monitors the light transmitted through the optical path turning part 140.