A substrate-type optical waveguide includes: a mode conversion section including a first input light guide path and a first output light guide path; and a polarization conversion and multiplexing section including a second input light guide path and a second output light guide path. The mode conversion section converts light input into the first input light guide path into either: a superposition of a first TE fundamental mode and a first TE higher mode; or a superposition of a first TM fundamental mode and a first TM higher mode. The polarization conversion and multiplexing section converts either: the superposition of the first TE fundamental mode and the first TE higher mode; or the superposition of the first TM fundamental mode and the first TM higher mode, into a superposition of a second TE fundamental mode and a second TM fundamental mode.