In order to provide a reciprocating optical modulation system capable of obtaining a broad bandwidth, a reciprocating optical modulation system (1) of the present invention is basically provided with an optical modulator (2) modulating an output light by controlling one of an intensity, a phase and a frequency of an input light; a first fiber grating (3) transmitting a light of a predetermined frequency domain among lights inputted to and outputted from the optical modulator while reflecting lights of other frequencies; a second fiber grating (4) transmitting a light of a predetermined frequency domain among lights inputted to and outputted from the optical modulator while reflecting lights of other frequencies; and a signal source (5) generating a modulating signal to be inputted to the optical modulator (2).