Abstract: A transmitter casing has an upper plate and a lower plate, which face each other. A rib is located to incline with respect to the lower plate between the upper plate and the lower plate. The rib prevents the tire from interfering to the casing if the tire is detached from the wheel. This enables smooth detaching of a tire from a wheel without interference with the transmitter, and to strengthen the casing of the transmitter.

Abstract: A signal transmitter mounted on a wheel of a vehicle is disclosed. The transmitter includes a stem member and a cap member being arranged to be detachably mounted on the stem member. The stem member serves as a tire valve when the cap member is detached from the stem member. The stem member detects an tire air pressure smaller than a predetermined level when the cap member is attached to the stem member. The cap member transmits a warning wave signal based on the detection of the stem member. The stem member has an air passage extending in the accommodating member from the open end to the closed end. An electrical conductor is disposed in the air passage to be electrically connected to the cap member when the cap member is mounted on the stem member. A pressure chamber is separated by a partition form the air passage. A tire air introducing passage communicates with the air passage and the tire at a point closer to the open end than the pressure chamber.