To provide an internal pressure alarm device capable of improvement of deterioration of a transmission characteristic because radio waves radiated from an antenna are affected by potential fluctuation of a rim and widening of a propagation range of radio waves, in a tire internal pressure alarm device mounted on the rim, detecting internal pressure of a tire and transmitting a detected signal from the antenna to a receiver on a vehicle body side.

A casing of the tire internal pressure alarm device is provided with a cover part on an opposite side of a bottom surface side of a well part of the rim of an electronic board at a mounting attitude on the rim. A casing antenna part constituting a part of the antenna is provided at the inside or on the surface of the cover part.