A frictional engagement apparatus is provided. The frictional engagement
apparatus includes a first frictional engagement unit, a second
frictional engagement unit disposed axially spaced from the first
frictional engagement unit, and one case that supports the first and the
second frictional engagement units non-rotatably circumferentially but
shiftably axially relative thereto. One retaining ring is fitted to the
case so as to be located between the first and second frictional
engagement units. The retaining ring is abutted to the nearest first
friction plate and/or the second friction plate to stop an approaching
shift of the first frictional engagement unit and/or the second
frictional engagement units. The retaining ring includes a u-shaped cross
section defined by an annular recess that extends in a circumferential
direction of the retaining ring at a radially interior portion of the
retaining ring.