A clutching device of an automatic transmission may include: a clutch drum connected with a first rotation element, a clutch connected with the clutch drum and also connected with a second rotation element for selective torque delivery, and a piston unit to operate the clutch. In particular, the piston unit includes: a first piston moving toward the clutch by a hydraulic pressure; a spring retainer arranged on the first rotation element; a support drum arranged in the first piston; a second piston arranged in the support drum and moving in an axial direction of the first rotation element; a first return spring applying restoring force to the first piston; a second return spring applying restoring force to the second piston; a latch spring applying pressure to the clutch when the first piston moves toward the clutch; and a latch device selectively limiting bidirectional movement of the first piston.