A hydraulic system for a construction machine, the hydraulic system comprising: a manipulation device (1, 20) for traveling and a manipulation lever for a work apparatus that are configured to output manipulation signals in proportion to a manipulation amount; first and second hydraulic pumps (3, 4); a left traveling motor connected to the first hydraulic pump and driven by the manipulation of a manipulation device for left traveling; a first control valve mounted in a discharge flow path of the first hydraulic pump and configured to control a start, a stop, and a direction change of the left traveling motor when it is shifted; a right traveling motor connected to the second hydraulic pump and driven by the manipulation of a manipulation device for right traveling; a hydraulic actuator connected to the first hydraulic pump or the second hydraulic pump, and driven by the manipulation of the manipulation lever for the work apparatus; a second control valve mounted in a discharge flow path of the first hydraulic pump or the second hydraulic pump, and configured to control a start, a stop, and a direction change of the hydraulic actuator when it is shifted; a third control valve mounted in a flow path branched from the discharge flow path of the second hydraulic pump and configured to control a start, a stop, and a direction change of the right traveling motor when it is shifted; a first bypass valve connected to the upstream side of the discharge flow path of the first hydraulic pump and configured to be controlled in an opening amount based on the manipulation amount of the manipulation device for left traveling or the manipulation lever for the work apparatus; a second bypass valve connected to the upstream side of the discharge flow path of the second hydraulic pump and configured to be controlled in an opening amount based on the manipulation amount of the manipulation device for right traveling or the manipulation lever for the work apparatus; a confluence valve mounted in a flow path which connects the discharge flow paths of the first and second hydraulic pumps (3, 4) in parallel, and configured to be controlled in an opening amount based on the manipulation amount of the manipulation devices (1, 20) for traveling or the manipulation lever for the work apparatus; and a controller; characterized in that the controller is configured to control the opening amounts of the first and second bypass valves (11, 12) and the confluence valve in response to the input of the manipulation signals of the manipulation devices (1, 20) for traveling and the manipulation lever for the work apparatus, and when a combined operation of the left and right traveling motors and the work apparatus are performed, the opening areas of the first bypass valve and the second bypass valve are controlled to be equal to each other and the opening amount of the confluence valve is controlled to be the maximum. a manipulation device (1, 20) for traveling and a manipulation lever for a work apparatus that are configured to output manipulation signals in proportion to a manipulation amount; first and second hydraulic pumps (3, 4); a left traveling motor connected to the first hydraulic pump and driven by the manipulation of a manipulation device for left traveling; a first control valve mounted in a discharge flow path of the first hydraulic pump and configured to control a start, a stop, and a direction change of the left traveling motor when it is shifted; a right traveling motor connected to the second hydraulic pump and driven by the manipulation of a manipulation device for right traveling; a hydraulic actuator connected to the first hydraulic pump or the second hydraulic pump, and driven by the manipulation of the manipulation lever for the work apparatus; a second control valve mounted in a discharge flow path of the first hydraulic pump or the second hydraulic pump, and configured to control a start, a stop, and a direction change of the hydraulic actuator when it is shifted; a third control valve mounted in a flow path branched from the discharge flow path of the second hydraulic pump and configured to control a start, a stop, and a direction change of the right traveling motor when it is shifted; a first bypass valve connected to the upstream side of the discharge flow path of the first hydraulic pump and configured to be controlled in an opening amount based on the manipulation amount of the manipulation device for left traveling or the manipulation lever for the work apparatus; a second bypass valve connected to the upstream side of the discharge flow path of the second hydraulic pump and configured to be controlled in an opening amount based on the manipulation amount of the manipulation device for right traveling or the manipulation lever for the work apparatus; a confluence valve mounted in a flow path which connects the discharge flow paths of the first and second hydraulic pumps (3, 4) in parallel, and configured to be controlled in an opening amount based on the manipulation amount of the manipulation devices (1, 20) for traveling or the manipulation lever for the work apparatus; and a controller;