An optical transmission device of the invention has an optical transmitter unit 1 in which a light emitting element and a light receiving element are sealed with resin by transfer molding, and a holder 2 which holds the optical transmitter unit 1 being contained therein and which holds an optical plug being fitted therein with an optical fiber cable fixed to the optical plug. The holder 2 holds the optical plug that has been inserted from a direction generally at right angles to optical axes of the optical transmitter unit 1. The optical transmission device can easily be mounted on a printed board without necessity of a dead area for insertion/extraction of optical plug and can be small in size and thickness.