Abstract: An auto-drainage assembly to be inserted into a pneumatic piping system which supplies pressurized air to a pneumatically operating machine, for separating a drain liquid from the piping. The auto-drainage assembly is the type which includes a drain reservoir case, a hollow discharge guide shaft fitted in position at the bottom of the reservoir case, a drain valve seat provided within the discharge guide shaft for discharging the drain liquid to the outside, a hollow piston driven by pressurized air supplied to a pilot chamber in relation with upward and downward movements of a float within the drain reservoir case, and a drain valve body provided on the piston. The drain valve body is constituted by an annular seal member which is fitted on the outer periphery of the piston, and a large number of drain discharge grooves are provided on the inner periphery of the discharge guide shaft in such a way as to define a corresponding number of guide surfaces between the respective drain discharge grooves.