The present invention provides an apparatus, method and program for alarming abnormality in tire air-pressure which prevent the issuance of an erroneous alarm and thus determine abnormality in tire air-pressure even when a rotational velocity of the tire changes by causes other than a decrease in an air pressure. An apparatus for alarming abnormality in tire air-pressure decrease detects a change in a relationship between wheel velocities of front wheel tires and those of rear wheel tires, and replaces a relationship between the wheel velocities of the front wheel tires and those of the rear wheel tires with a relationship between wheel velocities of front wheel tires and those of rear wheel tires obtained after the change of the relationship, thereby determining abnormality in air-pressure when the change is detected.