A transmitter 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 comprising: a battery, which is configured to serve as a power source for the transmitter; a condition detecting section (24, 27), which is configured to detect a condition of the transmitter; a trigger receiving section, which is configured to receive, 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 is configured to transmit a signal; a controlling section, which is configured to perform 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 is configured to store at least one of reception time of the unmodulated wave and a number of times of the response transmission; the transmitter being characterized by comprising a traveling detecting section, which is configured to detect traveling and stopping of the vehicle, wherein the controlling section is configured to determine whether to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and cause the transmitting section to transmit the signal to a receiver mounted on the vehicle when traveling of the vehicle is detected after determining not to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and the signal includes data configured to cause the receiver to recognize that the unmodulated wave received when the vehicle is at a stopped state is not regarded as the trigger signal. a battery, which is configured to serve as a power source for the transmitter; a condition detecting section (24, 27), which is configured to detect a condition of the transmitter; a trigger receiving section, which is configured to receive, 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 is configured to transmit a signal; a controlling section, which is configured to perform 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 is configured to store at least one of reception time of the unmodulated wave and a number of times of the response transmission; the transmitter being characterized by comprising a traveling detecting section, which is configured to detect traveling and stopping of the vehicle, wherein the controlling section is configured to determine whether to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and cause the transmitting section to transmit the signal to a receiver mounted on the vehicle when traveling of the vehicle is detected after determining not to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and determine whether to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and cause the transmitting section to transmit the signal to a receiver mounted on the vehicle when traveling of the vehicle is detected after determining not to regard, as the trigger signal, the unmodulated wave that is received by the trigger receiving section when the vehicle is at a stopped state, and the signal includes data configured to cause the receiver to recognize that the unmodulated wave received when the vehicle is at a stopped state is not regarded as the trigger signal.