Provided is a breather pipe structure for a liquid reducing agent storage tank which is capable of introducing/discharging air according to changes of liquid amount in the liquid reducing agent storage tank and capable of controlling the full-tank liquid surface level during injection of a liquid reductant. In the breather pipe structure for a liquid reducing agent storage tank (10) which stores a liquid reducing agent (5) and which has a breather pipe (20) attached on top of the tank for introducing/discharging air, a lower end (20c) of the breather pipe (20) extending downward from the top of the liquid reducing agent storage tank (10) is positioned at a level corresponding to the liquid surface level when the tank is full.