Provided is a urea water consumption amount diagnostic device for urea SCR which calculates an appropriate urea water consumption amount taking into account quantization errors of a urea water sensor due to replenishment. Based on the calculated amount, the urea water consumption amount diagnostic device performs a comparison with a cumulative injected amount, which is injected from a dosing valve, to detect abnormality in the dosing valve and/or other components. The urea water consumption amount diagnostic device includes a urea water sensor (22) which detects a level of urea water in a urea water tank (15), a consumed amount calculation means (32) which calculates a cumulative consumed amount (D) by calculating the cumulative consumed amount (D) based on a detection value entered from the urea water sensor (22) and calculating the replenishment amount (R) based on the change in level of the urea water sensor (22) after replenishment of the urea water, and an abnormality diagnostic means (33) which, when th
