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1. A tire-state detection device (10C), comprising:

a sensor (410) for detecting a predetermined physical state of a tire;

an antenna (470) having a predetermined resonance frequency and circling around a predetermined axis circumference at a predetermined diameter, the antenna having a coiled shape extending in an axial direction;

a detection circuit (400) for transmitting information regarding a result of a detection made by the sensor from the antenna as radio waves; and

a case (100) for housing the sensor, the antenna and the detection circuit, and for allowing radio waves to pass therethrough,

the case being fitted on a rim (3) in the tire when the tire-state detection device is to be used;

a printed wiring board (300C) secured on the case is provided in the case;

the antenna (470) is securely provided to one surface of the printed wiring board; characterized in that

there is provided a planar conductor (361) secured in a state of being electrically insulated from the antenna at a position set a predetermined distance away from the antenna so as to form an interface between the antenna and the rim when the case is fitted to the rim, and set to a potential that is equivalent to a reference potential of the detection circuit; and

the planar conductor (361) is secured to the printed wiring board by a plurality of holders (371) so that another surface of the printed wiring board and one surface of the planar conductor are parallel at a predetermined distance, and so that a predetermined space is formed between another surface of the printed wiring board and one surface of the planar conductor.

  2. The tire-state detection device according to claim 1, wherein the axis of the coil-shaped antenna (470) is arranged parallel to a surface of the planar conductor.