A master apparatus repeatedly executes an offset value calculation notification sequence in which a calculation of an offset value for correcting a transmission path delay is implemented after communications with the plurality of slave apparatuses, and a notification of the offset value calculated to the plurality of slave apparatuses is implemented Every time the offset value calculation notification sequence is executed, the master apparatus notifies the slave apparatuses of a sequence identifier which is the identifier of the offset value calculation notification sequence. Furthermore, the master apparatus selects a sequence identifier from among two or more sequence identifiers already notified to each slave apparatus, notifies the selected sequence identifier to each slave apparatus, and instructs each slave apparatus to use, for time-synchronization, an offset value notified in the offset value calculation notification sequence corresponding to the selected sequence identifier.