A plunging assembly of a driveshaft comprising: 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, characterized in that the outer housing has a mounting hole communicating with the bore, wherein 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, wherein the outer housing has an inclined surface connecting the bore and the mounting hole, wherein the plug member comprises: a shock absorbing portion located in the bore, a fastening portion inserted into the mounting hole, and a connecting portion connecting the shock absorbing portion and the fixing portion, and wherein an outer peripheral surface of the connecting portion is formed to be inclined so as to be in close contact with the inclined surface. 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, characterized in that the outer housing has a mounting hole communicating with the bore, wherein 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, wherein the outer housing has an inclined surface connecting the bore and the mounting hole, wherein the plug member comprises: a shock absorbing portion located in the bore, a fastening portion inserted into the mounting hole, and a connecting portion connecting the shock absorbing portion and the fixing portion, and wherein an outer peripheral surface of the connecting portion is formed to be inclined so as to be in close contact with the inclined surface. a shock absorbing portion located in the bore, a fastening portion inserted into the mounting hole, and a connecting portion connecting the shock absorbing portion and the fixing portion, and wherein an outer peripheral surface of the connecting portion is formed to be inclined so as to be in close contact with the inclined surface.