Abstract: The direct control of a change in output amplitude of the modulator driver resulting from changes in the ambient environment or by aging is difficult because deterioration is generated in the drive waveform under the condition that the drive signal of the radio frequency is output from the modulator driver of the Mach-Zehnder modulator. A low frequency element included in the modulator output is monitored by superimposing the low frequency signals under the in-phase condition to two envelopes corresponding to the H input and L input of the drive signal. Amplitude of the drive signal outputted from the modulator driver can be adjusted without deterioration of the drive waveform.