Abstract: In an air breather structure in a transmission, a partition wall 2 defining an air breather chamber S extends substantially vertically downward in a curved configuration at a distance from a corner portion 1a of a transmission case to increase volume of the air breather chamber S. An oil entrance 4 is formed in an upper portion of the partition wall 2. An oil exit 5 wider than the oil entrance 4 is formed in a lower end portion of the partition wall 2 for discharging oil which enters the air breather chamber S. The oil exit 5 is formed between a lower end of the partition wall 2 and an oil-returning rib 6 projecting from the transmission case for preventing the oil from flowing upward into the air breather chamber S. With this construction, the air breather structure becomes free from "blow of oil".