Patent ID: 8248224

Claim:
An apparatus for detecting a decrease in a tire air pressure, comprising: a rotation information detection unit configured to periodically detect tire rotation information regarding the respective wheels of a four-wheel vehicle; a wheel speed calculation unit configured to calculate tire wheel speeds based on the rotation information detected by the rotation information detection unit; a comparison value calculation unit configured to calculate a difference between a sum of wheel speeds of two wheels at one diagonal positions and a sum of wheel speeds of two wheels at the other diagonal positions as a first comparison value and for calculating a difference between a sum of wheel speeds of two wheels at one same side positions and a sum of wheel speeds of two wheels at the other same side positions as a second comparison value; a determination unit configured to determine a tire having a decreased pressure based on a relation between the first comparison value and/or the second comparison value and a predetermined determination threshold value; and an alarming unit configured to issue an alarm when the determination unit determines a tire having a decreased pressure, wherein the apparatus further comprises a conversion unit configured to convert the first comparison value and the second comparison value to a plane for which change amounts of the first comparison value and the second comparison value due to a decreased pressure from the normal value learnt during a time of a normal pressure are unified between front wheels and rear wheels, and the determination unit is so configured as to perform a decreased pressure determination by a single determination threshold value.