An optical substrate includes: a submount; a planar optical element which is mounted on the submount; a pair of positioning members which are disposed at an interval across the planar optical element on the submount; an optical waveguide in which a core and a clad are formed by a flexible material; and a holding member which holds the optical waveguide to allow a tip end of the optical waveguide to be inserted between the pair of positioning members, and the optical waveguide to be bent to extend parallel to the submount.