To cause an internal pressure alarm device for a tire which detects an internal pressure of the tire, and issues an alarm when the internal pressure is abnormal, to judge that a vehicle is in course of running even when the vehicle frequently repeats its runs and stops, and to rapidly judge that the vehicle is in a running state when the vehicle starts after a long stop.

When the vehicle is judged not to be in course of running, if there is an interruption by a second timing signal of a period longer than a first timing signal for periodically measuring the internal pressure of the tire, data on the internal pressure of the tire are transmitted without hoisting a running flag. If there are no interruptions by the second timing signal, and no running flags are hoisted either, a running flag is hoisted and data on the internal pressure of the tire are transmitted, if the vehicle is judged to be in course of running. When the vehicle is judged not to be in course of running, no running flags are hoisted, and data on the internal pressure of the tire are not transmitted either.