A new wavelength-selective optical modulator was proposed and discussed. The modulator consists of three kinds of distributed Bragg reflectors (DBRs) integrated in a single straight waveguide. The waveguide can guide TE₀ and TE₁ modes, and an in-line Michelson interferometer is constructed by the three DBRs. An operation-wavelength wave among incident wavelength-division-multiplexed TE₁ guided waves is split into TE₀ and TE₁ guided waves by one of DBRs, and combined by the same DBR to be TE₀ output wave with interference after one of waves is phase-modulated. A modulator using an electro-optic (EO) polymer is designed, and the static performance was predicted theoretically. An operation principle was confirmed experimentally by a prototype device utilizing a thermo-optic effect instead of the EO effect.