Arranged for at least one of a pair of branch optical waveguides in a Mach-Zehnder type interference optical system is a ring resonance type phase shifter for modulating a light wave signal propagating through the branch optical waveguide. The ring resonance type phase shifter includes a ring-type optical waveguide arranged so as to be mode-coupled with the corresponding branch optical waveguide, and is configured so that amplitude branching ratio K between the corresponding branch optical waveguide and the ring-type optical waveguide can be varied with a change in refractive index or the like, accompanied by voltage application to a pn junction, for example. As amplitude branching ratio K is varied, the phase difference between the light wave signals propagating through the paired optical waveguides varies, to thereby control the intensity of the light wave signal output from the interference optical system.