Abstract: In the case of an OAM cell for each of N paths being generated and transmitted at a rate of one OAM cell per a predetermined time period of 1 second in each of the N paths, first, N subperiods is prepared which are allocated to the N paths, respectively, and are obtained by dividing the predetermined time period by N. From the N subperiods, a single subperiod is sequentially selected. A cell stream is checked for an idle cell and the OAM cell for a single path of the N paths corresponding to the single subperiod selected is checked whether it is requested to be inserted into the cell stream within the single superiod selected. When an idle information block is first detected and the OAM cell is requested to be inserted into the cell stream, the OAM cell is inserted into the first idle cell which is first detected within the single subperiod selected.