A urea SCR system includes a tank that stores urea water, an injector that injects urea water to exhaust gas, a connection passage that connects the tank and the injector, an electric pump that is arranged at the connection passage and delivers urea water from the tank toward the injector or from the injector toward the tank, and a control device that controls the electric pump and the injector. The control device executes a suction-back operation for driving the electric pump so that urea water contained in the injector is suctioned back to the tank. Further, the control device determines whether or not the injector is stuck closed.