A plug of a connector that performs optical transmission is provided with an optical connection unit. The optical connection unit includes a lens that is provided on a side surface of the optical connection unit and performs optical connection with a lens provided on an optical connection unit of a receptacle in a direction orthogonal to an inserting/removing direction of the plug, and a locating surface that is provided on the side surface of the optical connection unit and abuts on a locating surface of the receptacle to determine a position of the optical connection unit in a mating condition of the plug into the receptacle.