A hydraulic circuit for a construction machine, comprising: first and second hydraulic pumps (2, 3) and a pilot pump connected to an engine; a first hydraulic actuator (5, 6) connected to the first hydraulic pump through a first center bypass path; a second hydraulic actuator connected to the second hydraulic pump through a second center bypass path; an operation lever outputting an operation signal corresponding to an operation amount during an operation by an operator; a main control valve having spools which are shifted by pilot signal pressure that is supplied from the pilot pump through the operation of the operation lever so as to control flow directions of hydraulic fluid supplied from the first and second hydraulic pumps (2, 3) to the first and second hydraulic actuators, respectively; and a first center bypass valve installed to be opened and closed on a downstream side of the first center bypass path in the main control valve, and shifted to intercept returning of the hydraulic fluid from the first hydraulic pump to a hydraulic tank when the first center bypass valve is shifted to a closed state; characterized in that a second center bypass valve is installed to be opened and closed on a downstream side of the second center bypass path in the main control valve, and shifted to intercept returning of the hydraulic fluid from the second hydraulic pump to the hydraulic tank when the second center bypass valve is shifted to a closed state, a switching valve is installed to be opened and closed in a signal path between the pilot pump and the first and second center bypass valves (14, 15), wherein when the switching valve is shifted to an opened state to make the first and second center bypass valves (14, 15) shifted to the closed state by the pilot signal pressure supplied from the pilot pump through the switching valve, the first and second center bypass paths (7, 9) in the main control valve are intercepted, and the hydraulic fluid from the first and second hydraulic pumps (2, 3), which has been increased up to relief pressure, is returned to the hydraulic tank by a main relief valve of the main control valve, and the switching valve is arranged on an upstream side or on a downstream side of a safety solenoid valve that is shifted to an on state when a safety lever mounted on a side surface of an operator's seat is operated. first and second hydraulic pumps (2, 3) and a pilot pump connected to an engine; a first hydraulic actuator (5, 6) connected to the first hydraulic pump through a first center bypass path; a second hydraulic actuator connected to the second hydraulic pump through a second center bypass path; an operation lever outputting an operation signal corresponding to an operation amount during an operation by an operator; a main control valve having spools which are shifted by pilot signal pressure that is supplied from the pilot pump through the operation of the operation lever so as to control flow directions of hydraulic fluid supplied from the first and second hydraulic pumps (2, 3) to the first and second hydraulic actuators, respectively; and a first center bypass valve installed to be opened and closed on a downstream side of the first center bypass path in the main control valve, and shifted to intercept returning of the hydraulic fluid from the first hydraulic pump to a hydraulic tank when the first center bypass valve is shifted to a closed state; characterized in that a second center bypass valve is installed to be opened and closed on a downstream side of the second center bypass path in the main control valve, and shifted to intercept returning of the hydraulic fluid from the second hydraulic pump to the hydraulic tank when the second center bypass valve is shifted to a closed state, a switching valve is installed to be opened and closed in a signal path between the pilot pump and the first and second center bypass valves (14, 15), wherein when the switching valve is shifted to an opened state to make the first and second center bypass valves (14, 15) shifted to the closed state by the pilot signal pressure supplied from the pilot pump through the switching valve, the first and second center bypass paths (7, 9) in the main control valve are intercepted, and the hydraulic fluid from the first and second hydraulic pumps (2, 3), which has been increased up to relief pressure, is returned to the hydraulic tank by a main relief valve of the main control valve, and the switching valve is arranged on an upstream side or on a downstream side of a safety solenoid valve that is shifted to an on state when a safety lever mounted on a side surface of an operator's seat is operated.