A driving apparatus supplies a drive signal to an optical modulator corresponding to a phase modulation format, and also, divides a part of an output light from the optical modulator; extracts some optical component of the divided light, which is distant from a carrier frequency by integral multiple of a modulation frequency, to detect a power of the optical component; and feedback controls a duty ratio of the drive signal or a cross-point level thereof so that the detected power approaches a minimum value. Thus, it is possible to stably perform phase modulation with high precision.