In a projector or an optical receiver, a master device produces pieces of definition information that determines timings of detection processes in the device including its own device and transmits the pieces of definition information to the slave device. The master device simultaneously transmits commands to require operations of internal timers to be synchronized with each other to the other slave devices in response to elapse of a time corresponding to a period in which a circulating period of the detection processes of the sensors proceeds by a predetermined cycle. Each of the slave devices corrects the timer in response to the command, and determines a timing at which a detection process should be performed with reference to a time point for correction on the basis of the definition information matched with its own device to execute the detection process.