To reduce loads of an operator in excavating work by detecting the tension of support ropes and controlling it between the lower limit value and the upper limit value in the period of operating the support hydraulic winch in the lowering direction.

When the rope tension detected by a load meter is determined to be a lower limit value or less, the lower side electrode valve 35L is turned on, and the control valve 12 is shifted from the position L side to the neutral position N, so that the lowering speed slows down and the rope tension increases. On the other hand, when the rope tension detected is beyond the lowering limit value, the lowering side electrode valve 35L is turned off, and the control valve 12 is shifted toward the position L side so as to be increased in its opening area, thus, the lowering speed slows down so that the rope tension decreases. When the detected value of rope tension is in the region between the lower limit value and the upper limit value, the electrode valve 35L is kept off. In this instance, pilot pressure in a pipe passage 33L accords to the manipulated variable of a pilot operating device 32.