Abstract: When a protocol signal is transmitted between a facsimile machine which operates under new ITU-T Recommendation and another facsimile machine which operates under old ITU-T Recommendation, a certain functional bit to specify a particular function of the new facsimile machine may not be understood by the old facsimile machine if such a functional bit is previously included in a protocol signal for non-standard procedure but now included in a protocol signal for standard procedure. If the old facsimile machine does not interpret the functional bit, a desired communication cannot be conducted between the two facsimile machines. To avoid such a case, a prescribed functional bit is included in both a protocol signal for standard procedure and a protocol signal for non-standard procedure. The functional bit included in the protocol signal for non-standard procedure will be understood by the old facsimile machine.