When the sub-heaters of recording heads are PWM controlled in a low-temperature environment, temperatures of the recording heads are respectively detected by temperature sensors. The number of recording heads to be heating controlled is determined from the detected recording head temperatures, and a maximum duty ratio of a pulse train supplied to the sub-heater corresponding to the recording head to be heating controlled is determined in accordance with the determined number of the recording heads, so that supply timings of pulse width controlled pulse trains supplied to the sub-heaters corresponding to the recording heads to be heating controlled are not overlapped.