The invention is capable of minimizing a phase shift of first and second oil holes when optionally installing a second drum in a first drum without indiscriminately increasing the number of oil holes. This pressure oil supply device of an automatic transmission supplies pressure oil to a hydraulic servo of a second clutch C-4 via a pressure oil supply passage 60 to the first drum 31 and the second drum 32. The pressure oil supply passage 60 is composed of a plurality of first oil holes 65 formed at an equal interval in the peripheral direction in the first drum 31, a plurality of second oil holes 66 at an equal angle interval on the circumference formed at an equal interval in the peripheral direction in the second drum 32 and an annular oil passage 67 formed in the first sand second drums and mutually communicating the first oil hole with the second oil hole. The number of other oil holes is increased more than the number of oil holes formed in one.