A method of calculating deflection of a rotating tire, through high speed data processing, based on measurement time series data of acceleration on a tire tread portion has the following steps. A step of acquiring measurement time series data of acceleration on a tire circumference in a radial direction of a rotating tire; a step of extracting acceleration data in a vicinity of a contact region from the acquired measurement data and subjecting the extracted acceleration data to least-square regression using a second-order differential function expression of a deflection function expression that represents a peak shape having a peak value and a curve gradually approaching to zero on both sides thereof, whereby a deflection function parameter values are determined; and a step of calculating a contact deflection from the deflection function expression determined by the deflection function parameter values.