A phase modulation apparatus has a light source outputting continuous light, two phase modulators, and an intensity modulator. The phase modulation apparatus is provided with an RZ phase modulation circuit, in which the phase modulators phase-modulate the continuous light from the light source with data signals input to the phase modulators and generate two phase modulation optical signals, a phase shifter shifts the phase of one phase modulation optical signal by π/2, and an intensity modulator intensity-modulates a multiplexed signal, combined with the other phase modulation optical signal, with an input clock signal CLK to convert the signal into an RZ signal, and, thus, to output the RZ signal, and a phase control circuit which adjusts the phases of the phase modulation optical signals generated by the phase modulator of the RZ phase modulation circuit so that the output of the RZ phase modulation circuit is maximum.