A plunging assembly of a driveshaft includes: an outer housing having a bore having a plurality of outer ball grooves extending in a longitudinal direction; an inner shaft being disposed to be able to undergo relative movement in the longitudinal direction in the bore of the outer housing and having a plurality of inner ball grooves that are paired respectively with the outer ball grooves to form a plurality of ball tracks; a plunging unit connecting the outer housing and the inner shaft to allow a plunging motion and a rotational power transmission between the outer housing and the inner shaft; and a stopper that is provided in the bore of the outer housing to limit relative movement of the inner housing and the inner shaft in a direction in which the inner shaft is inserted into the bore. The outer housing has a mounting hole communicating with the bore, and the stopper is a plug member that is inserted into the mounting hole in a state that a frontal end portion thereof is exposed to the bore.