PURPOSE: To transmit even one page image in which much quantity of data is contained at high speed by simultaneously using the two lines of the B channels of an ISDN.

CONSTITUTION: When image information is transmitted between terminals connected with an ISDN having two B channels for data transmission and a D channel for a control signal, the both of the two B channels are secured at the both terminals, the data for each page of the image information to be transmitted is decomposed into plural blocks, identifier is attached to each block, and they are distributed and transmitted by simultaneously using the two B channels on a transmission side. On a reception side, image information is restored by combining each data block received simultaneously from the two B channels in accordance with the identifier attached to it.