To detect the ground contact length of a wheel tire without installing on the wheel side, a power source requiring troublesome maintenance such as charging, and with a constitution having the small number of parts to be installed on the wheel side.

A piezoelectric element 11 for generating a voltage by deformation is buried in a part deformed by ground contact of the wheel tire 6, and a conductive wire 12 for generating a magnetic field by feeding from the piezoelectric element 11 is buried elongatively over the whole circumference of the wheel tire 6, and a magnetic field sensor 13 for generating a current by detecting the magnetic field of the conductive wire 12 is mounted on the body side. A controller 16 measures a time interval from generation of the change resulting from start of the ground contact of the tire 6 to generation of the change resulting from finish of the ground contact based on a current value from the magnetic field sensor 13 relative to each wheel, and determines the ground contact length of the tire from the ratio between the time interval and a time required for making one revolution of the tire determined from the wheel velocity of the corresponding wheel, and a tire radius. Abnormal lowering of the tire pressure is checked by determining whether the ground contact length of the tire is over an air pressure lowering determination value or not.