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.