To sufficiently show drilling performance by communicating a directional changeover valve with an operation oil tank by means of a regenerating spool valve, arranging a changeover means for changing over the regenerating spool valve according to pump pressure of a main pump and thereby directly feeding return oil from a rod side oil chamber of an arm cylinder directly to the operation oil tank under heavy load.

A regeneration joining releasing oil passage position is determined at a return oil passage opening oil passage position where a return oil passage (a) is communication with an operation oil tank 45 at a hydraulic cylinder extention operation changeover position of a pilot changeover valve 37 for an arm. Return oil from a rod side oil chamber 36 of an arm cylinder 34 is fed back to directly to the operation oil tank 45 through a pipe passage 57, the return oil passage (a) of the pilot changeover valve 37 for the arm, a pipe passage 58, the regeneration joining releasing oil passage position, and a pipe passage 59. Since back pressure hardly occurs in the rod side oil chamber 36 of the arm cylinder 34, drilling performance of an arm 29 is shown by 100 percent under heavy load.