Provided is a vehicle-mounted system capable of transmitting a command to a first system from a second system during a period in which the first system communicates with the second system according to a unidirectional communication protocol. A vehicle-mounted system 100 includes a sensor 10 (first system) and an ECU (second system). The sensor 10 outputs a message signal including a pause pulse to the communication line DATA according to SENT (a unidirectional communication protocol). The ECU 20 is connected to the communication line DATA, and transmits a command to the sensor 10 using the falling period of the pause pulse.