To provide a tire-state determination device capable of detecting the state of a tire irrespective of the state of a road surface.

This tire-state determination device includes wheel speed sensors (22, 24, 26, 28) for detecting the rotational speed of a tire and a control unit 40 which frequency-analyzes the result of the detection by the wheel speed sensors (22, 24, 26, 28) to detect a frequency at the valley portion of an output, and which calculates the pneumatic pressure of the tire from the detected frequency at the valley portion.