An input light-pulse (Pi), which were input to a transmitting-type diffraction grating (20) at a prescribed incident angle, are dispersed for each of the wavelengths and output at exiting angles corresponding to the wavelengths, are input to the transmitting type diffraction grating (20) at incident angles corresponding to the wavelengths after being reflected successively at reflecting mirrors (41, 42, 43), and output at a prescribed exiting angle from the transmitting type diffraction grating (20). Each of the wavelength-component light-rays, which were output at a prescribed exiting angle from the transmitting type diffraction grating (20), have the optical paths thereof turned back by a right-angled prism (40), are input to the transmitting type diffraction grating (20) at a prescribed incident angle and output at exiting angles corresponding to the wavelengths, and input to the transmitting type diffraction grating (20) at incident angles corresponding to the wavelengths after being reflected successively by the reflecting mirrors (43, 42, 41). Light-rays input to the transmitting type diffraction grating (20) at incident angles corresponding to the wavelengths are multiplexed by the transmitting type diffraction grating (20), and output as an output light pulse (Po). In such a way, a pulse-width converting apparatus and an optical amplifying system that can easily be made to be compact are achieved.