An urea water supply system for supplying urea water into an exhaust passage of an internal combustion engine, the exhaust passage being provided with an exhaust emission control device having a first NOx catalyst and a second NOx catalyst configured to reduce NOx by using ammonia as a reducing agent, the urea water supply system comprising: a first supply valve that is located in upstream of the first NOx catalyst and is configured to supply urea water to an exhaust emission flowing into the first NOx catalyst; a second supply valve that is located in upstream of the second NOx catalyst and is configured to supply the urea water to the exhaust emission flowing into the second NOx catalyst; a urea water tank that is configured to store the urea water; a urea water supply path (L1, L2, L3) that is arranged to connect the urea water tank with each of the first supply valve and the second supply valve and includes a first supply path which only the urea water to be supplied to the first supply valve flows through and a second supply path which only the urea water to be supplied to the second supply valve flows through, wherein the second supply path has a larger capacity than capacity of the first supply path by a predetermined volume; and a pump that is configured to pressure-feed the urea water in the urea water supply path (L1, L2, L3); characterized by a controller that is configured to perform suck-back control of the urea water in the first and second supply valves (6, 16) and in the urea water supply path (L1, L2, L3) by operating the pump in a specified operating state, opening the first supply valve for a first estimated valve-opening time that is a valve-opening time of the first supply valve estimated to be required for the suck-back control of the urea water in the first supply valve and in the first supply path and opening the second supply valve for a second estimated valve-opening time that is a valve-opening time of the second supply valve estimated to be required for the suck-back control of the urea water in the second supply valve and in the second supply path, the controller performing open-close control of the first supply valve and the second supply valve in the suck-back control such that the first estimated valve-opening time is shorter than the second estimated valve-opening time by at least a first control time corresponding to the predetermined volume. a first supply valve that is located in upstream of the first NOx catalyst and is configured to supply urea water to an exhaust emission flowing into the first NOx catalyst; a second supply valve that is located in upstream of the second NOx catalyst and is configured to supply the urea water to the exhaust emission flowing into the second NOx catalyst; a urea water tank that is configured to store the urea water; a urea water supply path (L1, L2, L3) that is arranged to connect the urea water tank with each of the first supply valve and the second supply valve and includes a first supply path which only the urea water to be supplied to the first supply valve flows through and a second supply path which only the urea water to be supplied to the second supply valve flows through, wherein the second supply path has a larger capacity than capacity of the first supply path by a predetermined volume; and a pump that is configured to pressure-feed the urea water in the urea water supply path (L1, L2, L3); characterized by a controller that is configured to perform suck-back control of the urea water in the first and second supply valves (6, 16) and in the urea water supply path (L1, L2, L3) by operating the pump in a specified operating state, opening the first supply valve for a first estimated valve-opening time that is a valve-opening time of the first supply valve estimated to be required for the suck-back control of the urea water in the first supply valve and in the first supply path and opening the second supply valve for a second estimated valve-opening time that is a valve-opening time of the second supply valve estimated to be required for the suck-back control of the urea water in the second supply valve and in the second supply path, the controller performing open-close control of the first supply valve and the second supply valve in the suck-back control such that the first estimated valve-opening time is shorter than the second estimated valve-opening time by at least a first control time corresponding to the predetermined volume.