For an optical transmitter in which an EA modulator modulates a laser beam emitted from an LD, a control circuit stops heating/cooling of the EA modulator when a casing temperature (TC) is within a range of a low-temperature side reference temperature (T_cool) and a high-temperature side reference temperature (T_heat), sets a bias voltage for the EA modulator to a bias voltage corresponding to the modulator temperature based on table information recorded on a memory circuit, heats the EA modulator and sets the bias voltage corresponding to the low-temperature side reference temperature when the casing temperature is equal to or lower than the low-temperature side reference temperature, and cools the EA modulator and sets the bias voltage corresponding to the high-temperature side reference temperature when the casing temperature is equal to or higher than the high-temperature side reference temperature.