To realize an automatic threshold changing operation of pneumatic tire pressure upon completion of a pressure adjustment under a tire rotation or its replacement work in an irregular pneumatic tire pressure detecting device.

Whether or not a tire pressure adjustment is carried out is judged based on the result of detection performed by a valve core opening or closing detection means for use in detecting the fact that an opening or closing operation of the valve core installed for performing a tire pneumatic pressure adjustment is carried out. In the case that it is judged that the tire pressure adjustment is carried out, a threshold value of the tire pneumatic pressure for each of the wheels is set. Then, an irregular tire pneumatic pressure of the wheel fixed to a vehicle is detected based on the result of comparison between the detected tire pneumatic pressure and its set threshold value.