To provide an apparatus for adding a proper amount of particulate additive material to each of mud plugs different in size, which are conveyed as a large number of blocks with air spaces interposed in front and rear positions of them in a carrier pipe.

A squeeze pump 3 makes the slurry mud sent out into the carrier pipe 1 for pneumatically pumping the slurry mud; the sent-out slurry mud is pneumatically pumped in the state of being divided into the large number of massive mud plugs; the slurry mud S, which is discharged to the outlet side of the carrier pipe 1 from an outlet 1a of the carrier pipe 1, is discharged into a mud swirling guide 30 which has a ring-shaped floor portion overhanging along an inner surface of a cylindrical peripheral wall for guide and which has a mud dropping opening in its central portion; the particulate additive material B is supplied onto the ring-shaped floor portion; and the slurry mud S and the particulate additive material B are mixed together by being swirled in the mud swirling guide 30.