To keep communication even if a distance between transmission devices is farther and a transmission distance therebetween is longer.A transmission device for alternately performing transmission and reception to/from a communication destination transmission device transmits transmit data to the communication destination transmission device. It receives return data transmitted by the communication destination transmission device after the transmit data reaches the communication destination transmission device. A predetermined period of time is assumed as one time unit, one transmission of transmit data and one reception of return data are tried within the one time unit, and when it is determined that one transmission of transmit data and one reception of return data do not fall within the one time unit, the transmission device is controlled such that N number of transmit data are continuously transmitted and then N number of return data are continuously received within N (N is an integer of two or more) time units.