To automatically determine the completion of the jackup of an outrigger mounted on a construction machine such as a track crane by accurately detecting the jackup.

Pressure accumulators 20a to 20d are connected to the head sides of right and left suspension cylinders 16a to 16h by oil passages 21a to 21d, and opening and closing valves 23a to 23d are interposed. The rod side of each suspension cylinder 16a to 16h is connected to a tank 24 by oil passages 25a, 25b, and an opening and closing valve 26 is interposed on the side of the tank 24. A pressure switch 27 is provided on the rod side of the suspension cylinder 16b. To jack up an outrigger during operation, at first the opening and closing valves 23a to 23d and 26 are closed to block each oil passage. Under this state the jackup is started, and when the wheels leave the ground, all the load of nonsuspended weight is applied to the rod side of each suspension cylinder 16a to 16h, the pressure switch 27 is turned on, and a computer judges that the jackup is complete.