PURPOSE: To secure the responsiveness of a higher speed if the reception processing ability has its surplus margin by controlling the idle time secured in a transmission period in accordance with the reception processing ability.

CONSTITUTION: A transmission system defines one of plural transmission stations 1 as a master station and other stations 1 as the slave stations respectively. These stations 1 are connected together via a transmission cable 3. When a transmission period is started by a medium access control means of the master station, the slave stations detect the start timing and transmit it in the order of station addresses. Thereafter, the transmission period is repeated so that each station 1 performs the cyclic transmission in a fixed time. In such a constitution of a data transmission method, the data processing part of each station 1 receives an actuation start signal and then outputs the next transmission period start signal to the medium access control means of the master station after the data processing is completed. Thus a transmission period optimum to the data processing time can be selected.