The improved quadrature phase shift keying modulator has a structure such that the average light output power of phase-shift keying modulation light output from the combining unit is changed according to the phase difference between the first and the second optical signal after being combined by means of applying driving signals different in eye crossing percentage by the first modulator and the second modulator, respectively, and has a power monitor monitoring the average light output power of quadrature phase-shift keying modulated light and a phase shift controlling unit which performs feedback control of the phase shift amount in the phase shifting unit based on the average light output power monitored by the power monitor.