PURPOSE: To aim at improvements in steerability and stability for driving a car, by taking frequency components equivalent to both springing and unspringing resonant values individually out of the vibration inputted into the car, while comparing them with each of the specified reference values, and making it so as to judge a road state according to size in the comparison.

CONSTITUTION: On the basis of vibration to be transmitted to a car body, a road state sensor 1 makes a road surface detection signal DS input into each of processing circuits 30 and 40. The circuit 30 outputs an unspringing road surface signal RS1 upon comparing an unspringing road state signal, proportional to size of the surface unevenness, with the unspringing reference voltage Rref1. Likewise, the circuit 40 outputs a springing road state signal RS2. A damping force regulating actuator 56 is driven via microcomputer 65 and a drive circuit 60, while a damping force select signal is inputted into a judging level setting circuit 70 whereby the damping force is selected where it should be, according to the road state. With this constitution, steerability and stability for driving a car are improvable.