A disturbance estimating observer is provided, which estimates a load applied to a servomotor for driving an ejector axis. When an ejector operates, the disturbance load torque estimated by the observer is sampled for each specific period (0 to j), and the data DA (0, 0) to DA (i, j) of (i+1) ejector processes are stored in a table TA in the form of cyclic storage. From this table TA, average values DB (0) to DB (j) of estimated disturbance load torques are determined in turn and they are stored in a table TB. From these average values, upper limit values DC (0) to DC (j) and lower limit values DD (0) to DD (j) of a permissible range are stored in tables TC, TD. When the determined estimated disturbance load torque has exceeded the permissible range, an abnormal load signal is outputted.