An automatic transmission including a double pinion planetary gear unit PG1, two single pinion planetary gear units PG2 and PG3 and six frictional elements (viz., six clutches) C1 to C6 is so constructed as to establish nine forward speeds and one reverse speed by simultaneously putting selected three of the six frictional elements C1 to C6 into their engaged condition. The frictional elements C1 to C6 are of a type including mutually rotatable friction plates that cause production of an oil dragging resistance when the frictional element is under idling condition. With the simultaneously engaged condition of the three frictional elements, each speed is established with a reduced friction loss of the automatic transmission.