Pickup coils which generate vehicle wheel speed signals which include vibration frequency components of tires during vehicle cruising are provided. An electronic control unit extracts resonance frequencies of tires from the vehicle wheel speed signals and estimates tire pneumatic pressure based on the extracted resonance frequencies. When an estimated vehicle wheel pneumatic pressure falls below a predetermined critical pneumatic pressure, pneumatic pressure decline warnings are generated and displayed by a display unit. An update switch is provided and when this update switch is turned on when replacing tires, the electronic control unit adjusts and sets the value of the critical pneumatic pressure described above based on the resonance frequency extracted during this time.
