To remarkably enhance the error correcting capability if the speed of information data increases or the transmission speed increases by increasing the ratio of redundant data for information data and a deterioration of optical transmission characteristics increases.

This device is provided with: a first multiplexing/demultiplexing circuit 11 and a second multiplexing/demultiplexing circuit 12 which generate the parallel data by multiplexing/demultiplexing the STM-64 data input; a frame generating FEC encoding circuit 13 performs an error correcting code calculation in the direction of sequence of matrix which the parallel data forms, adds the error correcting code as the result of calculation to the parallel data, performs the error correcting code calculation in the direction to go of matrix which the parallel data forms, adds the error correcting code as the result of calculation to the parallel data; the first multiplexing/demultiplexing circuit 14 and the second multiplexing/demultiplexing circuit 15 which multiplex the parallel data applied error correcting code and output as FEC frame.