An automatic transmission supplies oil pressure to a hydraulic servo of a second clutch through an oil pressure supply passage extending through a first drum and a second drum, the oil pressure supply passage including a plurality of first oil holes formed in the first drum at equal, circumferential intervals, a plurality of second oil holes formed in the second drum at equal, circumferential intervals, and an annular oil passage formed between the first and second drums and connecting the first oil holes and the second oil holes. The number of first oil holes is different from the number of second oil holes.