The present invention relates to an optical signal processor which can perform favorable optical signal processing even when there are environmental changes and the like. The optical signal processor inputs light emitted from an end face of an optical fiber, 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; and comprises optical systems, a diffraction grating device, mirror reflectors, an optical path turning part, and a monitor part. The optical path turning part transmits therethrough a part of the incident light and reflects at least a part of the remnant. The optical system monitors the light transmitted through the optical path turning part.