The device control system includes: a device controller connected to a load device via a signal line; a wireless master connected to the signal line; and a wireless slave. The device controller outputs a first control signal indicative of an intended state of the load device when receiving a second control signal from the wireless slave via the wireless master or receiving via the signal line a third control signal sent in response to a state check signal. When receiving a state change notice sent in response to the first control signal, the device controller sends the state change notice to the wireless master. When receiving the state change notice, the wireless master sends the received state change notice to the wireless slave.