A method for estimating the magnitude of a tire force acting on a rolling tire is disclosed, wherein: using at least seven sensors mounted on a tire sidewall portion, surface strain is simultaneously measured at seven measuring positions; with respect to a combination of the seven measuring positions, a coefficient matrix representing correlation with seven force components is defined, and a equation including the coefficient matrix is defined; using the data on the surface strain measured with the sensors, the equation is computed to find at least one of the seven force components, e.g. air pressure. The estimated air pressure is compared with a reference pressure, to determine whether the tire pressure is dropped or not. Further, a secondary sensor is mounted in a peculiar region where the surface strain is substantially not affected by the vertical load.