The present invention provides a single fiber bidirectional optical transmission system capable of realizing the extension of a single fiber bidirectional long distance at a moderate price.An optical signal outputted from a second optical transmitter is incident on an optical amplifying portion passing through an optical circulator, a single fiber bidirectional transmission path, an optical circulator and an optical Blue/Red filter. The optical signal outputted from a first optical transmitter is incident on the optical amplifying portion passing through a dispersion compensator and the optical Blue/Red filter. By inserting the dispersion compensator immediately after the first optical transmitter, a difference between a power level by which the optical signal from the first transmitter is inputted to the optical amplifier and the power level by which the optical signal from the second optical transmitter is inputted to the optical amplifier is made small, so that, even when the optical amplifier is installed at one end of the transmission path, both of the bidirectional signals can obtain sufficient gains, thereby realizing a bidirectional collective amplification.