To provide an all-optical optical multi-level modulation signal generator generating a very high-speed optical multi-level modulation signal with stable operation without requiring complicated high-accuracy control technology.

A signal light of linear polarization as an optical pulse train is divided into first and second components as two linear polarization components, and the first and second components are input to a closed-loop optical path so that they circulate in the opposite directions. In this closed-loop optical path, two optical phase modulation parts individually changing the optical phase of the first component and that of the second component according to first and second control optical signals having intensity patterns in accordance with signal sequences are provided. An optical phase difference application part applying a relative optical phase difference to the first component and the second component is also provided in a certain position on the optical path. A pair of the first and second components output from the closed-loop optical path and provided with varied optical phases, having relative optical phase differences applied thereto, is output as a multi-level modulation optical signal.