To simplify the makeup of a hydraulic circuit in a hydraulic excavator with a multistage extensible arm for deep hole excavation.

A direction control valve 27 which controls both pressure oil to the working cylinder 25 of an extension arm and pressure oil to the working cylinder 26 of a clamshell bucket and an operating valve 28 are provided on the main machine 21 side of a hydraulic excavator. A switchover control valve 37 which supplies and drains pressure oil, switching from oil channels 33, 34 leading to the working cylinder 25 to oil channels 35, 36 leading to the working cylinder 26 and vise versa, is provided on the extension arm 23 side, and connected to the direction control valve 27 on the main machine 21 side with shared oil channels 38, 39. Depending on whether an operation switch 31 is on or off, an electromagnetic valve 29 is switched to (a) or (b), supplying pressure oil to the working cylinder 25 or 26 through the oil channels 33, 34 or 35, 36, and extending or retracting the multistage extensible am or opening or closing the clamshell bucket.