(EN) According to the present invention, when clogging of a filter (5) of an air cleaner is determined on the basis of an intake negative pressure of an engine (2) and the air filter (5) is cleaned, sequential measurement is performed by taking, as a cleaning interval (S), an accumulation value of the operation time of the engine (2) from the previous cleaning until the clogging is determined. A plurality of stages are set by discerning from the minimum value to the maximum value of the cleaning interval (S) in advance, and the measured cleaning interval (S) is added to a stage to which each stage belongs. A histogram is constructed by taking a transverse axis as a stage and a vertical axis as the frequency of the cleaning interval (S), and an approximate straight line (L2) is obtained in which the characteristic line (L1) of the histogram, which is represented with a dotted line, is approximated to a straight line to be represented with a solid line. The characteristics of the histogram change according to the deterioration progress of the air filter (5), and air filter deterioration is determined at a time point when the slope of the approximate straight line (L2) based thereon gradually decreases from sloping upward, which is shown with a thin line, to exceed (deltaY/deltaX=0) corresponding to the horizontal level.
(ZH) 在基于发动机(2)的进气负压作出空气净化器的过滤器(5)的堵塞判定并对空气过滤器(5)清扫时，将从上次的清扫到作出堵塞判定为止的发动机(2)的运转时间的累积值作为清扫间隔(S)依次测量。预先将清扫间隔(S)的最小值到最大值进行划分而设定多个等级，将测量到的清扫间隔(S)添加到各等级中所属的等级。生成以横轴为等级并以纵轴为清扫间隔(S)的频率的直方图，使虚线所示的直方图的特性线(L1)近似于直线并求出实线所示的近似直线(L2)。直方图的特性随着空气过滤器(5)的劣化发展而变化，基于此的近似直线(L2)的斜率从细线所示的右上升逐渐减少，在超过相当于水平的(ΔY/ΔX＝0)的时刻做出空气过滤器劣化的判定。
