To provide a monitoring system for tire air pressure, accurately detecting tire air pressure of each wheel while having simplified configuration.

The monitoring system for tire air pressure includes sensor units U1 to U4 each having a valve, disposed separately on the wheels W1 to W4, respectively, and the system detects air pressure through the sensor units U1 to U4, and wirelessly transmits detection signals including information of the detected tire air pressure from the sensor units U1 to U4 to an in-vehicle device 10. Resistance values of four caps 31 mounted on the valves of the wheels are set to four kinds of resistances R1 to R4, respectively. The sensor units U1 to U4 include information of the resistance values of the caps 31 in the detection signals. The in-vehicle device 10 determines which one of the wheels W1 to W4 the information of tire air pressure included in the detection signals represents about the tire air pressure, based on the information of the resistance included in the detection signal.