Apparatus, method and program for detecting decrease in tire air pressure

An apparatus for detecting a decrease in a tire air pressure includes: a rotation information detection means for periodically detecting tire rotation information regarding the respective wheels of a four-wheel vehicle; a wheel speed calculation means for calculating tire wheel speeds based on the rotation information; a comparison value calculation means for calculating 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 means for determining a tire having a decreased pressure; and an alarming means. The apparatus further includes a conversion means for converting 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 means is so configured as to perform a decreased pressure determination by a single determination threshold value.