A first wavelength converter of a wavelength-conversion device includes: an optical amplifier amplifying an incident light beam; a first dispersion flat fiber spreading the wavelength spectrum width of the amplified beam; and a first wavelength filter transmitting a predetermined wavelength bandwidth of the first fiber output beam. A center wavelength of the first wavelength converted beam is shifted by Δλ1 from that of the incident light beam. A second wavelength converter of the device does not include an optical amplifier and includes: a second dispersion flat fiber spreading the wavelength spectrum width of the first wavelength converted beam; and a second wavelength filter transmitting a predetermined wavelength bandwidth of the second fiber output beam. A center wavelength of the second wavelength converted beam is shifted by Δλ2 from that of the first wavelength converted beam. The Δλ1 and Δλ2 satisfy Δλ1+Δλ2=Δλ, Δλ1×Δλ2