In a supervisory control system where an upper supervisory control terminal supervises and controls transmitting devices, the upper supervisory control terminal has two supervisory states. The transmitting devices store state change information to be transmitted to the upper supervisory control terminal with an issue of the information being suppressed when the upper supervisory control terminal is in one supervisory state, and merge the state change information into a single message to be transmitted when the upper supervisory control terminal shifts to the other supervisory state.