PROBLEMSTo provide a method for evaluating characteristics of MZ interferometers in an optical modulator having a plurality of MZ interferometers.MEANS FOR SOLVING PROBLEMSWhen an optical modulator includes a plurality of MZ interferometers, the 0-degree component contains a signal derived from an MZ interferometer other than the MZ interferometers for evaluating the characteristic. For this, it is impossible to accurately evaluate the characteristic of the MZ interferometers. The present invention does not use the 0-degree component normally having the highest intensity. That is, the characteristic of the MZ interferometers are evaluated by using a side band intensity of the component other than the 0-degree component.