TIRE-STATE DETECTION DEVICE

Disclosed is a tire-state detection device which can not only acquire good antenna characteristics even if the shape of the rim to which the tire-state detection device is to be fitted changes, but can also be produced at lower costs and be mass produced. The tire-state detection device is provided with: a sensor (410) for detecting a predetermined physical state of a tire; an antenna (450) which has a predetermined resonant frequency; a detection circuit for transmitting information relating to the detection results of the sensor (410) as radio waves from the antenna (450); and a case (100) which houses the sensor (410), antenna (450), and detection circuit, and through which radio waves can pass, wherein said case (100) can be fitted to the rim (3) of a tire. The tire-state detection device is provided with a planar conductor (310) which is fit, in a state electrically insulated from the antenna (450), at a position away from the antenna (450) by a predetermined distance so as to form an interface between the antenna (450) and rim (3) when the case (100) is fit to the rim (3), and is set to a potential equivalent to the reference potential of the detection circuit.