An antenna 1 for a tire pressure information sending apparatus is
constituted by a circuit board provided with feeding electrodes and a
radiation electrode 8 having a loop surface substantially perpendicular
to the principal surface, the radiation electrode sending air pressure
information of a tire onto a principal surface of the circuit board by
radio waves. Thereby, when the apparatus is mounted on a wheel rim, a
magnetic field vector parallel to the principal surface of the circuit
board and parallel to the wheel rim is generated, and a direction of a
magnetic field vector by a mirror effect to the wheel rim becomes the
same as the direction of the magnetic field vector of the antenna. As a
result, it is possible to effectively prevent that the antenna
characteristic deteriorates.