PURPOSE: To inhibit small water-level variation as the disturbance of control while having control follow up the change of a water level comparatively rapidly.

CONSTITUTION: When smoothing is conducted by moving average by a moving- average processing means 4 on the basis of a water level data by a water lever recorder 1, either of the average number of data undergoing moving averaging or a sampling period is adjusted by a moving-average adjusting processing means 5 as required. Average number of data is increased or the sampling period is lengthened under the status, in which the water level is hardly varied, and the moving average time is lengthened. Average number of data is decreased or the sampling period is shortened under the status, in which the water level is fluctuated largely, and the moving average time is shortened. A gate machine side disc 2 is controlled through a control arithmetic processing means 6 and a control output section 7 on the basis of an optimum smoothed water level under the status.