A process bus-applied protection system includes a process bus, a plurality of MUs (merging units), and a plurality of IEDs (intelligent electric devices). Each of the MUs is configured to sample a current and a voltage of a power system at timing synchronized with a time synchronization signal received through the process bus. Each of the IEDs is configured to be capable of outputting the time synchronization signal to the process bus by serving as a transmission source, and receiving, through the process bus, the time synchronization signal from another IED. The plurality of IEDs have a predetermined priority. Each of the IEDs is configured, when the IED does not receive the time synchronization signal from an IED having a higher priority than that of the IED and serving as a transmission source, to output the time synchronization signal to the process bus by serving as a transmission source.