To provide a wheel load value calculating device capable of accurately calculating a value of a load applied to a tire based on the grounding area of the tire.

This wheel load value calculating device includes: sensor units 21a-21d for measuring pneumatic pressure of the tires Tia-Tid installed in wheels 20a-20d of a vehicle 1; piezoelectric elements 24a-24d for detecting a change in the circumferential direction and a change in the width direction of a tire load applied to the tires Tia-Tid when the vehicle 1 travels; and a control device 10 for calculating a tire load value. The control device 10 calculates the grounding area of the tires Tia-Tid based on the grounding length in the circumferential direction of the tires Tia-Tid and a grounding width in the width direction of the tires Tia-Tid, and also calculates the tire load value by using a correlation between the grounding area and the tire load value set with every pneumatic pressure of the tires based on a calculation value of the grounding area and a measured value of the tire pneumatic pressure.