1 shown in fig1 a modulation signal having an angular frequency u03c9 is applied to the modulator mz c and a modulation signal having an angular frequency 3 u03c9 which has offset by u03c0 2 90 degrees is applied to the other modulator mz s these modulation signals modulate input light and the optical outputs thus modulated by each modulation signal are mixed to have an opposite phase to that of signal components that inhibit the output to illustrate this more specifically it will be assumed that the following signals are applied to the terminals rf a sin u03c9t rf b cos u03c9t rs s u2212 sin 3 u03c9t and rf t cos 3 u03c9t here a bias voltage that produces the same phase relationship as that of the ssb modulator of fig1 is applied to terminals dc a and dc b and dc s and dc t in cases where there are a first order upper sideband and a third order lower sideband an analogous control can be used to suppress the third order lower sideband the constitution shown in fig2 can be simplified utilizing characteristics relating to superimposition of electromagnetic waves fig3 shows a preferred configuration thus simplified in this constitution the same.