To provide a means for buffering a digital optical signal, without having to use a long optical fiber loop.

The method for buffering digital optical signals (S(i), i=0,..., 3) with a prescribed digital level at least for a prescribed holding time, includes a step (a) where the optical signals S(i) is given to a semiconductor laser element(SLE(i)) and a step (b), where an injection current is injected to the semiconductor laser element(SLE(i)) to establish an optical gain process in the semiconductor laser element(SLE(i)). The injection current has an amplitude, so that the optical gain process and the optical absorption process in the semiconductor laser element(SLE(i)) overweight one another longer than the holding time, so as to maintain a digital optical signal to have a prescribed digital level for the holding time.