A transmitter is configured to be attached to each of wheel assemblies provided in a vehicle and execute a process in accordance with a command included in a trigger signal transmitted from a trigger device. The transmitter includes a battery, a condition detecting section, which detects a condition of the transmitter, a trigger receiving section, which receives, as the trigger signal, an unmodulated wave having a received signal strength indication greater than or equal to a predetermined received signal strength indication, a transmitting section, which transmits a signal, a controlling section, which performs response transmission, by which the signal is transmitted to the transmitting section in response to the unmodulated wave, when the trigger receiving section receives the unmodulated wave, and a memory section, which stores at least one of reception time of the unmodulated wave and a number of times of the response transmission.