A judgment whether a decrease in tire air-pressure occurs or not is performed by a process comprising the steps of: detecting information on wheel rotational velocities of tires; obtaining a judgment value for decrease in tire air-pressure based on relative comparison between the detected information on wheel rotational velocities; storing the judgment value(s) into an average preparation buffer BF; obtaining an average value of the judgment value(s) stored in the average preparation buffer BF when the vehicle stops; writing the average value and the number of the judgment values into a nonvolatile memory 2e; expanding the average value written in the nonvolatile memory 2e into the average preparation buffer BF in duplicate corresponding to the number of the judgment values after the start of a drive of the vehicle; and calculating an average value of a prescribed number of judgment values stored in the average preparation buffer BF so as to compare the average value with a threshold value.