An automatic transmission comprising: a transmission casing which houses therein a transmission mechanism, the transmission casing having: (a) a concave portion with which at least an oil inlet or an oil outlet communicates at an inside of the concave portion; and (b) an overhanging portion which is positioned close to the concave portion and extends along a surface of the transmission casing with a clearance provided between the surface and the overhanging portion, and a filter case which is attached to the transmission casing and has therein an oil filter, the filter case having; (c) a case body which is inserted into the concave portion; (d) a seal member which fits onto an outer circumference of the case body and seals a boundary between an outer circumferential surface of the case body inserted into the concave portion and an inside surface of the concave portion; (e) a first brim portion which protrudes from the outer circumferential surface of the case body and is fixed to the surface of the transmission casing; and (f) a second brim portion which protrudes from the outer circumferential surface of the case body and is inserted into the clearance. a transmission casing which houses therein a transmission mechanism, the transmission casing having: (a) a concave portion with which at least an oil inlet or an oil outlet communicates at an inside of the concave portion; and (b) an overhanging portion which is positioned close to the concave portion and extends along a surface of the transmission casing with a clearance provided between the surface and the overhanging portion, and (a) a concave portion with which at least an oil inlet or an oil outlet communicates at an inside of the concave portion; and (b) an overhanging portion which is positioned close to the concave portion and extends along a surface of the transmission casing with a clearance provided between the surface and the overhanging portion, and a filter case which is attached to the transmission casing and has therein an oil filter, the filter case having; (c) a case body which is inserted into the concave portion; (d) a seal member which fits onto an outer circumference of the case body and seals a boundary between an outer circumferential surface of the case body inserted into the concave portion and an inside surface of the concave portion; (e) a first brim portion which protrudes from the outer circumferential surface of the case body and is fixed to the surface of the transmission casing; and (f) a second brim portion which protrudes from the outer circumferential surface of the case body and is inserted into the clearance. (c) a case body which is inserted into the concave portion; (d) a seal member which fits onto an outer circumference of the case body and seals a boundary between an outer circumferential surface of the case body inserted into the concave portion and an inside surface of the concave portion; (e) a first brim portion which protrudes from the outer circumferential surface of the case body and is fixed to the surface of the transmission casing; and (f) a second brim portion which protrudes from the outer circumferential surface of the case body and is inserted into the clearance.