A method for estimating a road surface condition, comprising the steps of: detecting an unsprung fore-aft acceleration by an acceleration sensor attached to an unsprung position of a vehicle; detecting a wheel speed; calculating a variation in the detected wheel speed; calculating a fluctuation range of the calculated variation in wheel speed and a fluctuation range of the detected unsprung fore-aft acceleration; and estimating a road surface condition from a relationship between the fluctuation range of the variation in wheel speed and the fluctuation range of the unsprung fore-aft acceleration, wherein the step of estimating a road surface condition further includes the steps of: estimating whether the road surface under a traveling vehicle is a rough road surface or not from a relationship between the fluctuation range of the variation in wheel speed and the fluctuation range of the unsprung fore-aft acceleration; and calculating, when the estimated road surface is a rough road surface, a peak frequency at a peak position within a band of 200 to 230 Hz of a frequency spectrum obtained by a frequency analysis of the detected unsprung fore-aft acceleration and estimating whether the rough road surface is a slippery road surface or not from the peak frequency and the wheel speed, wherein in the step of estimating whether the road surface under a traveling vehicle is a rough road surface or not, the road surface under a traveling vehicle is estimated to be a rough road surface when the calculated fluctuation range of the unsprung fore-aft acceleration is greater than the calculated value of the fluctuation range of the unsprung fore-aft acceleration obtained by substituting the fluctuation range of the variation in wheel speed into a predetermined fluctuation range criterion formula representing a relationship between the fluctuation range of unsprung fore-aft acceleration and the fluctuation range of variation in wheel speed, and wherein in the step of estimating whether the rough road surface is a slippery road surface or not, the rough road surface is estimated to be a slippery road surface whose road surface friction coefficient μ is lower than 0.3 when the detected peak frequency is lower than the calculated value of peak frequency obtained by substituting the detected wheel speed into a frequency criterion formula representing a predetermined relationship between peak frequency and wheel speed. detecting an unsprung fore-aft acceleration by an acceleration sensor attached to an unsprung position of a vehicle; detecting a wheel speed; calculating a variation in the detected wheel speed; calculating a fluctuation range of the calculated variation in wheel speed and a fluctuation range of the detected unsprung fore-aft acceleration; and estimating a road surface condition from a relationship between the fluctuation range of the variation in wheel speed and the fluctuation range of the unsprung fore-aft acceleration, wherein the step of estimating a road surface condition further includes the steps of: estimating whether the road surface under a traveling vehicle is a rough road surface or not from a relationship between the fluctuation range of the variation in wheel speed and the fluctuation range of the unsprung fore-aft acceleration; and calculating, when the estimated road surface is a rough road surface, a peak frequency at a peak position within a band of 200 to 230 Hz of a frequency spectrum obtained by a frequency analysis of the detected unsprung fore-aft acceleration and estimating whether the rough road surface is a slippery road surface or not from the peak frequency and the wheel speed, estimating whether the road surface under a traveling vehicle is a rough road surface or not from a relationship between the fluctuation range of the variation in wheel speed and the fluctuation range of the unsprung fore-aft acceleration; and calculating, when the estimated road surface is a rough road surface, a peak frequency at a peak position within a band of 200 to 230 Hz of a frequency spectrum obtained by a frequency analysis of the detected unsprung fore-aft acceleration and estimating whether the rough road surface is a slippery road surface or not from the peak frequency and the wheel speed, wherein in the step of estimating whether the road surface under a traveling vehicle is a rough road surface or not, the road surface under a traveling vehicle is estimated to be a rough road surface when the calculated fluctuation range of the unsprung fore-aft acceleration is greater than the calculated value of the fluctuation range of the unsprung fore-aft acceleration obtained by substituting the fluctuation range of the variation in wheel speed into a predetermined fluctuation range criterion formula representing a relationship between the fluctuation range of unsprung fore-aft acceleration and the fluctuation range of variation in wheel speed, and wherein in the step of estimating whether the rough road surface is a slippery road surface or not, the rough road surface is estimated to be a slippery road surface whose road surface friction coefficient μ is lower than 0.3 when the detected peak frequency is lower than the calculated value of peak frequency obtained by substituting the detected wheel speed into a frequency criterion formula representing a predetermined relationship between peak frequency and wheel speed.