A receiving antenna RA of a receiving unit RU is positioned at an inner diameter side of a rotation locus of a transmitting unit TU when a tire T rotates. Consequently, it shortens a reaching distance of electromagnetic waves and lessens its change, thereby making it easy to keep a stable receiving sensitivity state. Moreover, the receiving antenna RA is made of a wire of a predetermined length that is made integral inside a wire harness WH connected to the receiving unit RU. Consequently, it becomes easy to make the receiving antenna RA in a length of λ/4 of a wavelength λ of the electromagnetic waves in use, a length of an integral multiple thereof, or ⅝λ. As a result, the receiving sensitivity can be improved.