An optical waveguide includes a clad layer, a core layer and a clad layer which are laminated together in this order from a lower side thereof. Within the core layer, a core portion and a side clad portion provided adjacent to the core portion so as to surround side surfaces of the core portion are formed. Further, a part of the side clad portion prevents a left side end of the core portion from being exposed outside. A mirror formation region is constituted from a region consisting of such a part of the side clad portion and a part of each of the clad layers located thereabove and therebelow. This mirror formation region is subjected to digging processing so that a concave portion is formed. An inner surface of this concave portion serves as the mirror. From the mirror, a material other than a material constituting the core portion, that is, a material constituting each of the clad layers and a material constituting the side clad portion are exposed.