A tire-state detection device including a sensor, an antenna having a predetermined frequency, a detection circuit, a case, and a planar conductor. The sensor detects a predetermined physical state of a tire. The detection circuit transmits information regarding a result of a detection made by the sensor from the antenna as radio waves. The case houses the sensor, the antenna and the detection circuit, and allows radio waves to pass. The case fits on a rim in the tire when the tire-state detection device is to be used. The planar conductor is 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 the planar conductor is set to a potential that is equivalent to a reference potential of the detection circuit.