PURPOSE: To dispense with adjusting operation of hitting strength of bucket which is difficult by operation of an operation means, eliminate unevenness on the finished surface even if the operation is done by an unskilled operator, and form high-quality compacted surface.

CONSTITUTION: When an operator presses down a soil compacting mode switch and then operates an operation lever which is provided newly forward, a controller drives a boom cylinder 8 at the maximum speed to output drive control signals which lower a boom 5. A microcomputer calculates x-z coordinate (PX, PZ) of a reference point P on the back of a bucket 7 based on joint angle data detected by a potentiometer provided in a joint section of a work machine 4 and judges whether it descends down to the deceleration start height d or not. If it is all right, it multiplies a ratio K of the height from virtual descent stop surface Sm as deceleration coefficient to operation signal of an operation lever to decelerate and lower a boom cylinder 8. At this time, when the operator operates a predetermined operation lever backward, drive control signals which decelerate the descent speed of the boom cylinder 8 in accordance with the operation amount of the operation lever are outputted from the controller.