In an actuator drive control device for an active vibration isolation support system, accumulated time of eight time intervals tn of crank pulses which are outputted at every 15 degrees of a crank angle in a vibration period corresponding to 120 degrees of the crank angle, and average accumulated time shown by a straight line connecting a start point and an end point of the line of the accumulated time are calculated. Eight deviations Δtn are calculated by subtracting the average accumulated time from the accumulated time. A variation waveform of the deviations Δtn corresponds to a variation waveform of the time interval tn of the crank pulses attributable only to the engine vibration with the influence of a variation in the engine rotational speed Ne being eliminated. Therefore, the vibration isolation function of an active vibration isolation support system can be effectively executed by controlling an actuator of the active vibration isolation support system based on the engine vibration which is estimated from the variation waveform of the time interval tn. Thus, the active vibration isolation support system is precisely controlled by accurately estimating a vibration state of the engine when the engine rotational speed increases and decreases.