A higher order mode generator includes a straight waveguide having upper and lower cladding layers and a core layer which support higher order modes higher than the 0th order. A linear heater is disposed across the waveguide at a tilt angle &agr; with respect to the waveguide. For converting the 0th order optical guiding mode into the mth order optical guiding mode, a tilt angle &agr; of the linear heater and the mth order mode propagation angle &thgr;m satisfy the condition: &agr;>&thgr;m/2.