A method for detecting decrease in tire air-pressure in which decrease in internal
pressure of a tire is detected on the basis of rotational information obtained
from tires attached to a vehicle. The method includes the steps of: detecting rotational
information of the respective tires; storing the rotational information of the
respective tires; obtaining a slip rate between the tires and a road surface during
running; comparing the slip rate with a slip rate which has been preliminarily
obtained when at normal internal pressure; and judging simultaneous decompression
of two wheel tires of both front wheels or simultaneous decompression of two wheel
tires of both rear wheels on the basis of a relationship between a value obtained
through comparison of the two slip rates and a specified threshold. It is possible
to judge simultaneous decompression of two wheel tires of both front wheels or
wheel two tires of both rear wheels.
