An air breather structure for an automatic transmission. The structure includes a transmission case housing a power train accommodating a plurality of friction engagement devices, and an extension case housing a rotational member. The extension case and the transmission case are united with each other to constitute an automatic transmission housing. A breather chamber is located upside the automatic transmission housing in a state where the automatic transmission is mounted in a vehicle, and formed with an air vent for communication with outside air. An end wall is provided for partitioning the transmission case and the extension case therebetween to define a transmission case-side space and an extension case-side space. In this structure, the automatic transmission housing is formed with a transmission case-side communication passage through which the breather chamber communicates with the transmission case-side space, and an extension case-side communication passage through which the breather chamber communicates with the transmission case-side space.