An automatic transmission has a transmission casing accommodating a transmission mechanism and a filter case attached to the transmission casing and having an oil filter. The transmission casing has a concave portion with which at least an oil inlet or an oil outlet communicates and an overhanging portion positioned close to the concave portion and extending along a surface of the transmission casing with a clearance provided between the surface and the overhanging portion. The filter case has a case body inserted into the concave portion, a seal member sealing a boundary between an outer circumferential surface of the case body and an inside surface of the concave portion, a first brim portion protruding from the outer circumferential surface and fixed to the surface of the transmission casing, and a second brim portion protruding from the outer circumferential surface and inserted into the clearance.