The invention relates to an outer-conical-surface supporting rotary tool device. The outer-conical-surface supporting rotary tool device comprises a bracket, outer rotary supporting wheel shafts, a rotary drive gear shaft, idle wheel shafts, an inner gear rack, a choke plug, a tool head and the like. The outer-conical-surface supporting rotary tool device is characterized in that the number of the outer rotary supporting wheel shafts is greater than or equal to 3; rotary center lines of all outer rotary supporting wheel shafts are on the same cylindrical surface; the inner gear rack and the choke plug are fixedly assembled together; an outer roller path on the inner gear rack supports on the outer rotary supporting wheel shafts; the two same idle wheel shafts are the same in structure; a gear on the rotary drive gear shaft is engaged with the gears on the two idle wheel shafts; gears on the two idle gear shafts are engaged with the inner gear on the inner gear rack; and the tool head is fixedly mounted on the outermost side in the radial direction of the inner gear rack. With the adoption of the outer-conical-surface supporting rotary tool device, a rotary tool obtains relatively high processing linear speed with relatively low rotation speed, breaks through the limit that a part must adopt a tool of the same size or a smaller size to process as a result of the small-size geometrical characteristic, and obtains better processing quality and a longer tool service life.