PURPOSE: To attain the switching to a different mode without change in the protocol procedure, by transmitting a part of a mode set signal together with a specific area in a picture transmission signal inclusive, in the transmission mode set system using the protocol procedure.

CONSTITUTION: An SK informing signal is inserted to a section tSKP at the transmission side every time a picture reading scanning section 1 detects a white line in the skip SK mode A set with the protocol procedure, and the line transmission period is brought to T'. A CPU4a discriminates whether or not the scanning line is the write line to be SK-processing depending whether or not the number of bits of the black part included in the scanning line reaches a prescribed number, and when the line is the SK line, data of synchronizing signal sections tSY and tSKP is added and transmitted. The transmission of data to an information storage section 10 is not done when the CPU4a detects the tSKP succeeding to the section tSY at the receiving side, a signal is given to a step motor 11 only and the line skip three times the normal frequency is done. In case of the SK mode B, a tSKPB is added to a period succeeding to the section tSKP to make the scanning line density of the SK line coarse.