A method, an apparatus and a program for alarming abnormality in tire air-pressure which enables to prevent issuing a erroneous alarm or no alarm when only one tire is replaced with a new tire can be provided. The method of alarming abnormality in tire air-pressure includes a step of detecting whether or not a new tire has been mounted by comparing the difference between determinate values to judge a decrease in air-pressure before and after enlargement of a diameter of the new tire with a set threshold value when solely one tire is replaced with the new tire with the use of a phenomenon that a diameter of a new tire on a vehicle enlarges after running.