To properly distinguish resonance frequencies of a tire from frequencies other than these, to enhance estimation precision for pneumatic pressure of the tire.

A wheel speed sensor 2 is arranged in each tire TR of a vehicle 1. Vibration components of a wheel speed caused by vibration of the tire are extracted from a wheel speed signal of the speed sensor 2 to find the resonance frequencies of the tire, and pneumatic pressure of the tire is detected based on the found resonance frequencies. A distribution of the resonance frequencies of the tire is calculated therein. A distribution condition in the resonance frequencies of the tire is determined, based on the calculated distribution, and detection for the pneumatic pressure of the tire based on the estimated resonance frequencies of the tire is inhibited, when the distribution condition is determined to be not uniform.