The determining method includes a step of continuously measuring the relative movement between the wheel (W F ) and the vehicle body (12) by use of a linear stroke sensor (28) while a motor vehicle is running; a step of creating spectral distribution by subjecting the output of the linear stroke sensor (28) to FFT frequency analysis; a step of calculating characteristic values (F) inherent to various types of road surfaces; and a step of specifying the type of the road surface based on the calculated characteristic values (F). When the type of the road surface is specified, the damping force of the suspension (10) is changed to a level corresponding to the type of the road surface. This level may be determined by a Fuzzy inference section (44).