A method and a device for manufacturing a rotary brush for roll tooth brush capable of easily removing plaque, providing an excellent massage effect on gums, capable of being mass-produced, and capable of reducing a cost by uniformizing the density of the bristles of the rotary brush (11), the method for manufacturing the rotary brush comprising the steps of projecting, by a specified amount, a wire group (1) formed of a large number of resin wires (1a) assembled together in a bundle to the outside through an insert hole (2a) provided in a pedestal (2), inserting a cone (4) to the projected tip center of the wire group (1) to push open the wire group (1) in radial directions, welding the center portion of the wire group (1) in an annular shape with the push-opened wire group (1) fixed onto the pedestal (2), cutting the inside of the welded part to form a sheet-like brush unit (8) having a hub (81) at the center thereof and having a large number of bristles (wires) (82) projected from the hub toward the radial outer side, holdingly inserting a core pipe (9) into the hub (81) of the brush unit (8), and taking out the brush unit (8) together with the core pipe (9) from the pedestal (2) to the outside.