A construction machine comprising a hydraulic system, comprising: first and second main pumps (11, 12) configured to discharge high-pressure hydraulic oil; a sub-pump configured to discharge pilot hydraulic oil; a first hydraulic line which is connected with the first main pump; a second hydraulic line which is connected with the second main pump; a traveling control valve unit which is disposed in the second hydraulic line, and configured to control a flow direction of the hydraulic oil that is configured to be supplied to a traveling motor; a first relief valve which is disposed in the first hydraulic line, and configured to maintain pressure of the first hydraulic line to a relief pressure or lower; a first control valve unit which is configured to be opened when a traveling mode is selected, and allow the pilot hydraulic oil to pass through the first control valve unit; and a second control valve unit which is configured to be operated by the pilot hydraulic oil supplied via the first control valve unit, and connects the hydraulic oil discharged from the first main pump to the second hydraulic line, characterized in that the construction machine further comprises: a shuttle valve which is configured to allow the pressure of the pilot hydraulic oil passing through the first control valve unit and the relief pressure to compete at both ends of the shuttle valve, and connects relatively high-pressure hydraulic oil to a first regulator of the first main pump; and a third control valve unit configured to reduce pressure of the output hydraulic oil provided to the first regulator through the shuttle valve, when a traveling mode is selected and pump pressure of any one of the first main pump and the second main pump is higher than a predetermined pressure. first and second main pumps (11, 12) configured to discharge high-pressure hydraulic oil; a sub-pump configured to discharge pilot hydraulic oil; a first hydraulic line which is connected with the first main pump; a second hydraulic line which is connected with the second main pump; a traveling control valve unit which is disposed in the second hydraulic line, and configured to control a flow direction of the hydraulic oil that is configured to be supplied to a traveling motor; a first relief valve which is disposed in the first hydraulic line, and configured to maintain pressure of the first hydraulic line to a relief pressure or lower; a first control valve unit which is configured to be opened when a traveling mode is selected, and allow the pilot hydraulic oil to pass through the first control valve unit; and a second control valve unit which is configured to be operated by the pilot hydraulic oil supplied via the first control valve unit, and connects the hydraulic oil discharged from the first main pump to the second hydraulic line, characterized in that the construction machine further comprises: a shuttle valve which is configured to allow the pressure of the pilot hydraulic oil passing through the first control valve unit and the relief pressure to compete at both ends of the shuttle valve, and connects relatively high-pressure hydraulic oil to a first regulator of the first main pump; and a third control valve unit configured to reduce pressure of the output hydraulic oil provided to the first regulator through the shuttle valve, when a traveling mode is selected and pump pressure of any one of the first main pump and the second main pump is higher than a predetermined pressure. a shuttle valve which is configured to allow the pressure of the pilot hydraulic oil passing through the first control valve unit and the relief pressure to compete at both ends of the shuttle valve, and connects relatively high-pressure hydraulic oil to a first regulator of the first main pump; and a third control valve unit configured to reduce pressure of the output hydraulic oil provided to the first regulator through the shuttle valve, when a traveling mode is selected and pump pressure of any one of the first main pump and the second main pump is higher than a predetermined pressure.