A facsimile unit exhibits a function for stepping down a data transmission rate from initial transmission rates to a number of new transmission rates on the basis of a ratio between the total number of data frames which have been initially transmitted to a receiver and the number of data frames which have been requested by the receiver for a retransmission. In this manner, an optimum new data transmission rate can be determined accurately and quickly. A parameter memory which contains information relating to ranges of determination in which an error correction mode of the operation is to be used, can be preferably provided in a facsimile device which exhibits both the error correction mode (ECM) and a normal mode. In this type of construction, the ECM mode can then be set automatically. In such a facsimile device which has both an ECM and a normal transmission mode, a common buffer memory is provided for temporarily storing coded image information either in the ECM or in the normal transmission mode.
