In order to estimate a braking force gradient in a low slip region including a steady travelling region, there is provided a device comprising a wheel speed sensor for detecting a wheel speed in a predetermined sampling period and outputting time series data of the wheel speed as a wheel speed signal, a break point frequency estimating section for estimating a break point frequency in a gain diagram that represents a frequency response of a first order lag model that approximates a transmission characteristics from road surface disturbances to wheel speed, and a braking force gradient estimating section for estimating a braking force gradient with respect to the estimated break point frequency on the basis of the map showing a relationship between braking force gradient and break point frequency stored in advance.