Control circuit of construction machine

In a hydraulic excavator having a central opening in the loop, even in the case of changing the weight of the bucket can be ensured as in the case of using a standard bucket was adjusted operability. Respectively, for controlling the oil supplied from the hydraulic pump (28) to the boom cylinder, arm cylinder (25) or the like operated valve boom, arm operation valve (251) or the like, provided with an intermediate bypass line (Cb). (55) is provided with a stick retraction / meter load pressure compensating valve (42) at the inner side from the rod (25R) arm cylinder (25) to the tank (29) of the rod-side return line. The rod side (25R) arm cylinder (25) is provided with a pressure supplied to the rod side (25R) of the working oil pressure sensor for detecting (81). There is provided in the direction of the center bypass line on the downstream side of the pressure increasing operation valve (251) with the rise of the arm by the pressure sensor (81) detected by the pressure in the intermediate bypass line (Cb) electromagnetic proportional control release valve (85).