An optical transmission apparatus includes a transmission in-process signal generation circuit (51) for detecting a data transmission start designation signal serving as a conversion start trigger given when parallel data input from the equipment located on the data transmission side is converted into serial data by the parallel/serial conversion circuit (41) on the data transmission side, and for supplying a transmission in-process signal, representing that data transmission processing is performed, to the equipment on the transmission side during at least a period required until the data input from the equipment on the data transmission side is transmitted to the equipment on the data reception side.