A method for evaluating characteristics of an optical modulator including a first Mach-Zehnder interferometer; wherein the first Mach-Zehnder interferometer includes a branching section, two arms (6, 7) connected to the branching section, a coupling section connected to the two arms (6, 7), and electrodes for applying bias voltages and modulation signals to the two arms (6, 7); wherein the method comprises a step of applying a driving signal the angular frequency of which is ω m to the first Mach-Zehnder interferometer, a step of sweeping a bias voltage applied to the first Mach-Zehnder interferometer, a step of measuring the intensity of the signal output from the first Mach-Zehnder interferometer while the bias voltage is being swept, and a step of evaluating characteristics of the first Mach-Zehnder interferometer; wherein is a step using intensities of components other than the zeroth-order component and the step of sweeping the bias voltage sweeps the voltage in a range selected to match with an integral multiple of the bias half-wave voltage; characterized in that: the characteristics of the first Mach-Zehnder interferometer include the imbalance (η) of light amplitude between the two arms; the optical modulator further comprises a second Mach-Zehnder interferometer; the step of evaluating characteristics of the first Mach-Zehnder interferometer comprises: calculating the sideband intensities S 1,S 2,S 3,S 4 of the first-order component to fourth-order component in the output from the first Mach-Zehnder interferometer measured at the step of measuring output intensity, the angular frequency of the first-order component being ω 0 +ω m, ω 0 being the angular frequency of input light input into the first Mach-Zehnder interferometer, the angular frequency of the fourth-order component being ω 0 +4ω m; and the step of evaluating characteristics of the first Mach-Zehnder interferometer further comprises: calculating the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1 and calculating the characteristics by solving simultaneous equations of the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1. the characteristics of the first Mach-Zehnder interferometer include the imbalance (η) of light amplitude between the two arms; the optical modulator further comprises a second Mach-Zehnder interferometer; the step of evaluating characteristics of the first Mach-Zehnder interferometer comprises: calculating the sideband intensities S 1,S 2,S 3,S 4 of the first-order component to fourth-order component in the output from the first Mach-Zehnder interferometer measured at the step of measuring output intensity, the angular frequency of the first-order component being ω 0 +ω m, ω 0 being the angular frequency of input light input into the first Mach-Zehnder interferometer, the angular frequency of the fourth-order component being ω 0 +4ω m; and calculating the sideband intensities S 1,S 2,S 3,S 4 of the first-order component to fourth-order component in the output from the first Mach-Zehnder interferometer measured at the step of measuring output intensity, the angular frequency of the first-order component being ω 0 +ω m, ω 0 being the angular frequency of input light input into the first Mach-Zehnder interferometer, the angular frequency of the fourth-order component being ω 0 +4ω m; and the step of evaluating characteristics of the first Mach-Zehnder interferometer further comprises: calculating the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1 and calculating the characteristics by solving simultaneous equations of the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1. calculating the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1 and calculating the characteristics by solving simultaneous equations of the three ratios S 2 /S 1, S 3 /S 1 and S 4 /S 1.