A wavelength conversion device including a nonlinear medium having quantum dots uses a tunable laser as an excitation light source of optical four wave mixing, and performs wavelength conversion for a wide bandwidth signal light with optical four wave mixing. In a wavelength conversion system (40) any one of the signal light having a wavelength of λ1, λ2, ..., λn is supplied to the wavelength conversion system (40) which includes an optical amplifier (41) for amplifying the signal light with an amplification factor α, a tunable laser (42), and an optical amplifier (43) for amplifying an output laser light from the tunable laser (42) with an amplification factor β. The light amplified in the optical amplifiers (41, 42) is coupled in an optical coupler (45) and supplied via an optical circulator (46) and a polarization beam splitter (47) to the quantum dot optical amplifier (20). A 1/2 wavelength plate (48) is inserted in the optical port of the polarization beam splitter (48) from which the TM mode light is extracted and converted to a TE mode light.