An input light pulse Pi, input at a constant incident angle to a transmission-type diffraction grating 20, is dispersed according to the wavelengths to be output at output angles according to the wavelengths, to be reflected by reflecting mirrors 41, 42, and 43 in series, and thereafter, the light rays are input at incident angles according to their wavelengths to the transmission-type diffraction grating 20, to be output at a constant output angle from the transmission-type diffraction grating 20. The optical path for the light rays of respective wavelength components, output at the constant output angle from the transmission-type diffraction grating 20, is folded back by a rectangular prism 40, to be input at a constant incident angle to the transmission-type diffraction grating 20, and the light rays are output at output angles according to their wavelengths, to be reflected by the reflecting mirrors 43, 42, and 41 in series, and are thereafter input at incident angles according to their wavelengths to the transmission-type diffraction grating 20. The light rays, input at the incident angles according to their wavelengths to the transmission-type diffraction grating 20, are coupled by the transmission-type diffraction grating 20, to be output as an output light pulse Po. Thereby, realizing the pulse width conversion device and the optical amplifier system, which are easily downsized.