Abstract: A tire pressure detector of an automobile, comprises a magnet having N and S poles and rotated in accordance with a pneumatic pressure in a tire, the N and S poles being rotatable in reverse, detecting means, arranged opposite to the magnet, for detecting magnetism and outputting a voltage signal changed to different magnetism level in accordance with rotation of the magnet, and determining means for comparing the voltage signal with a positive or negative threshold value and determining whether the pneumatic pressure in the tire is within a suitable range, based on a polarity of the voltage signal represented by a comparison result.

Abstract: A tire inflation pressure sensor mounted on the outside of the wheel. The sensor housing includes a diaphragm, a cylinder, a magnet holder, a spring, a magnet holding member connected with a magnet, a guide member and a stopper. When the inflation pressure of a tire is normal, the magnet holding member stops at a position where the magnet indicates a north pole. When the inflation pressure goes down below a specified value, the magnet holding member travels a predetermined distance and at the same time the magnet holding member rotates at a predetermined angle indicating a south pole. This changeover of magnetic polarity is detected by an external magnetic induction element to operate a warning device.