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.