An image transmission system including an image processing device and a communicating device which are communicable with each other through an access point in an infrastructure mode through an “m” channel, wherein upon a communication failure in the infrastructure mode, the communication mode is switched from the infrastructure mode to an ad hoc mode in which the two devices are directly communicable with each other through an “n” channel. During the communication in the ad hoc mode, the “m” channel is scanned to detect the access point at a predetermined time interval, and upon detection of the access point through the “m” channel, the communication mode is switched back to the infrastructure mode.