A diagnostic device for a urea water supply system having a sensor for detecting a concentration of urea water includes an acquisition section that acquires a detected value of the sensor, a calculation section that calculates a NOx purification rate η by a selective reduction catalyst, and a diagnosis section that diagnoses a state of the urea water supply system. The diagnosis section is adapted to diagnose that the sensor is abnormal when only one of a first condition and a second condition is satisfied. The first condition is that the purification rate η calculated by the calculation section is greater than or equal to a reference purification rate. The second condition is that the detected value Cur acquired by the acquisition section is greater than or equal to a reference concentration.