An apparatus for manufacturing an individual brush unit constituting a rotary brush by being stacked in multiple layers, includes an anvil having an insertion hole through which a filament group passes, a filament feeding unit which feeds the filament group from a feeder including a ring cutter on a tip and holds the filament group while a predetermined amount of the filament group protrudes outward from the insertion hole of the anvil, and an ultrasonic welder including a horn having a horn tip portion which pushes and radially expands a protruding tip of the filament group and welds a central part of the expanded filament group, to form the individual brush unit. A core of the welded central part is sandwiched between the ring cutter and the horn tip portion and is cut out, to simultaneously form a shaft hole and separate the individual brush unit from the filament group.