PURPOSE: To make improvements in a feeling of driving comfortableness and drivability, by judging the occurrence of rolling on the basis of road surface states at both sides, while controlling a degree of roll rigidity, and constituting a setting value for judgment so as to be changed with the control signal.

CONSTITUTION: Each of detecting signals RL and RR out of road state detector 1L and 1R is inputted into a differential value output circuit 3 via each of processing circuits 2L and 2R and outputted to a signal processing circuit 4 as signal differences at both sides. This signal processing circuit 4 individually selects and extracts an unspringing resonance component and a springing resonance component and outputs each of signals VA and VB to a judging circuit 5. A roll judging circuit 5 compares these input signals VA and VB with the setting value and outputs each of spring constant command signal CS1WCS4 commensurate to the preset spring constant to an air pressure regulator 6, controlling each of variable rate springs 11aW11b, simultaneously changes the setting value via a voltage setting circuit 7 for judgment. Thus, a feeling of driving comfortableness is improved.