An optical system 111 collimates wavelength components of wavelengths .lamda.1-.lamda.3 emerging from an end face of optical fiber 11, a diffraction grating 121 separates them by wavelength, and an optical system 112 condenses them. The component of the wavelength .lamda.1 is focused at a focus position by the optical system 112 and diverges after the focus position. Then the component of the wavelength .lamda.1 is collimated by an optical system 113, travels via a diffraction grating 122, and is condensed by an optical system 114 to enter an end face of optical fiber 22. The component of the wavelengths .lamda.2, .lamda.3 condensed by the optical system 112 are reflected by reflecting portions 132, 133 set at their respective focus positions, are collimated by the optical system 112, travel via the diffraction grating 121, and are condensed by the optical system 111 to enter an end face of optical fiber.