Provided is a method for detecting decrease in an inner pressure of a tire with excellent accuracy by calculating accurate dynamic load radii by comparing the GPS speed information with tire rotational speed information collecting data only when a stable measurement of the dynamic load radii is possible, as well as by utilizing GPS positioning information for a running state of a vehicle, and an apparatus and a program thereof.The present invention is characterized in judging decrease in an inner pressure of a tire from change in size of dynamic load radii by comparing an effective dynamic load radius with an initial value preliminarily stored as a dynamic load radius at a normal inner pressure, wherein the effective dynamic load radius is obtained by calculating an apparent dynamic load radius of a tire at running by successively comparing a calculated speed of a vehicle by a GPS device, in which the speed of the vehicle at running is calculated from information obtained by the GPS device, with a rotational speed of the tire obtained by a tire rotational speed detection apparatus installed on tire rotational portions of four wheels of the vehicle, and judging the apparent dynamic load radius to be effective only when a running state is judged to be straight on a flat road at a constant speed from GPS positioning information.