To effectively prevent a deterioration of antenna characteristics when an antenna is attached to a wheel rim.

The tire air pressure information transmitter comprises a circuit board having a feeding electrode, and a radiation electrode for transmitting tire air pressure information onto a main surface of the circuit board as radio waves and having a loop plane that is nearly perpendicular to the main surface, includes a dielectric material and a magnetic material in the loop plane, comprises an antenna for a tire air pressure information transmitter, a transmission circuit for transmitting the tire air pressure information as radio waves and a battery for driving the transmission circuit, and uses a battery case that is made of a metal as a part of the radiation electrode. The transmitter is attached so that the loop plane is nearly perpendicular to the main surface of a conductor part of a wheel to which a tire is attached.

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