A hydraulic system for a construction machine, comprising: first and second variable displacement hydraulic pumps (1, 2) connected to an engine; first and second hydraulic actuators connected to the first and second hydraulic pumps (1, 2); main control valves installed in flow paths between the first and second hydraulic pumps (1, 2) and the first and second hydraulic actuators, respectively, and having spools (3, 3a, 4, 4a) shifted to control a start, a stop, and a direction change of the first and second hydraulic actuators; and first and second operation levers (5, 6) outputting operation signals in proportion to a user's operation amount, characterized in that the hydraulic system further comprises a control device limiting a flow rate that is applied to the spool of any one of the first and second hydraulic actuators that share the first and second hydraulic pumps (1, 2) according to a user's operation of the first or second operation lever (5, 6) for a preferential function; and a control unit controlling driving of the control device so as to shift to or release the preferential function according to a user's determination, wherein the control device comprises: a first pressure sensor detecting the signal pressure according to the operation of the first operation lever and transmitting a detection signal to the control unit; a second pressure sensor detecting the signal pressure according to the operation of the second operation lever and transmitting a detection signal to the control unit; a first electro proportional pressure reducing valve connected to any one of the spools (3, 3a, 4, 4a) on the second hydraulic pump side so as to reduce a secondary signal pressure that is applied to any one of the spools (3, 3a, 4, 4a) on the second hydraulic pump side in reverse proportion to the operation amount of the second operation lever detected when shifted to the preferential function and to output the signal pressure according to the operation amount of the first operation lever when released to the preferential function; and a second electro proportional pressure reducing valve connected to any one of the spools (3, 3a, 4, 4a) on the first hydraulic pump side so as to reduce a secondary signal pressure that is applied to any one of the spools (3, 3a, 4, 4a) on the first hydraulic pump side in reverse proportion to the operation amount of the first operation lever detected when shifted to the preferential function and to output the signal pressure according to the operation amount of the second operation lever when released to the preferential function. a first pressure sensor detecting the signal pressure according to the operation of the first operation lever and transmitting a detection signal to the control unit; a second pressure sensor detecting the signal pressure according to the operation of the second operation lever and transmitting a detection signal to the control unit; a first electro proportional pressure reducing valve connected to any one of the spools (3, 3a, 4, 4a) on the second hydraulic pump side so as to reduce a secondary signal pressure that is applied to any one of the spools (3, 3a, 4, 4a) on the second hydraulic pump side in reverse proportion to the operation amount of the second operation lever detected when shifted to the preferential function and to output the signal pressure according to the operation amount of the first operation lever when released to the preferential function; and a second electro proportional pressure reducing valve connected to any one of the spools (3, 3a, 4, 4a) on the first hydraulic pump side so as to reduce a secondary signal pressure that is applied to any one of the spools (3, 3a, 4, 4a) on the first hydraulic pump side in reverse proportion to the operation amount of the first operation lever detected when shifted to the preferential function and to output the signal pressure according to the operation amount of the second operation lever when released to the preferential function.