To provide a helical cutter-head equipped with a detachable cutting blade easy to replace on wearing out or breaking.

The cutter-head is equipped with a plurality of grooves set spaced along peripheral direction and adapted to receive the detachable cutter-head inserts 18, and detachably securing means for the cutter-head inserts in the grooves. The cutter-head inserts have insert body and two blade units. The insert body comprises raised portions having triangular fitting sections formed therein so as to have two inwardly facing flat portions, a hole to receive the detachably securing means for the cutter-head inserts between the inwardly facing flat portions in the grooves, and two bolt holes to secure the two blade units to the insert body. The blade units are triangular with cut-off corners having edges respectively.