A communication system including a parent station and a plurality of child stations, the parent station and the plurality of child stations being line-connected or loop-connected in a plurality of stages, and the parent station comprises a decision unit configured to decide, based on information representing states of the plurality of child stations, which one a first data relay method of transmitting data to a subsequent station in accordance with a clock reproduced from data received from a preceding station and a second data relay method of transmitting data to the subsequent station in accordance with a local clock generated in a local station should be employed by each of the plurality of child stations.