When a diffraction grating having a grating period of d and a diffraction order of m for an incident light having a wavelength interval &Dgr;&lgr;i between i'th and (i+1)'th channels is used, an optical path length between the diffraction grating and the photodetector is represented by L and a mean output angle is represented by &thgr;o, a pitch pi between the i'th and (i+1)'th photodetectors in the photodetector array satisfies the equation of pi=m&Dgr;&lgr;iL/d cos &thgr;o.