To prevent leaking of a liquid in a casing to the exterior in a breather plug carrying out atmospheric air release when an internal pressure of a casing such as a transmission case rises.

An atmosphere side valve seat 22 and a case side valve seat 23 are provided in a liquid passage 21 of a breather body 2, a ball valve body 3 is arranged in the liquid passage 21 in a state of a gap C1 being formed between a passage inner face 21a and it, and a structure is provided allowing movement of the ball valve body 3 between the atmosphere side valve seat 22 and the case side valve seat 23. Thus, the ball valve body 3 is normally seated in the case side valve seat 23 by self-weight to prevent intrusion of foreign matter into the transmission case from the outside, and when an case internal pressure rises, the ball valve body 3 separates away from the case side valve seat 23, and the case interior is opened to the atmosphere through the gap C1. When lubricating oil in the transmission case rises to the fluid passage 21, the ball valve body 3 is seated in the atmosphere side valve seat 22 to prevent leakage to the outside.