To provide an apparatus, method and program for alarming decrease in tire
air-pressure which enable to detect decrease in air-pressure of all of
four-wheels tires at reduced costs. The apparatus for alarming decrease
in tire air-pressure includes means of (1) detecting decrease in tire
air-pressure by measuring change of parameter in relation to resonance
condition in the tire, and (2) detecting decrease in tire air-pressure by
making relative comparison of dynamic loaded radii of tires, wherein the
means (1) is included in one to three-wheel tires on a four-wheel
vehicle.