Abstract: Whether a previously computed valve command value Y−1 is within a neutral zone ±&agr; is determine. If the determination result is “Yes”, processing to compute a valve command value for a neutral dead zone is executed. If the determination result is “No”, processing to compute a valve command value for a driving zone is executed. In the latter case, by using a valve command value X and the previously computed valve command value Y−1, it is determined in which one of acceleration, deceleration/stop, and lever-reversed condition is the operating status, and a maximum setting rate in one of acceleration, deceleration/stop, and lever-reversed condition is computed from a corresponding function &Dgr;Y=fmax1(X), etc. Then, a control signal is computed while restraining a change rate of the operational signal to be kept not more than the computed maximum change rate, and a flow control valve 3 is controlled in accordance with the computed control signal.