PURPOSE: To reduce frequency of exchanging a piercer bar cap, manpower and cost by preventing wear of the abutting surface of core metal of a piercer bar cap of an assel mill.

CONSTITUTION: In the piercer bar cap 1 provided with a columnar projection 6 on whose tip end the core metal 7 of hollow shell shape is mounted covering the projection, a core metal supporting part 2 by which the rear end face of core metal is supported around the base bottom part of the projection and a screw part 5 with whose rear part a mandrel bar is connected by a screw, a buried build-up part 3 of an alloy whose hardness is higher than the hardness of piercer bar cap is provided on the abutting face of core metal supporting part 2 and the core metal is supported on the build-up part. A stellite of Co-Cr- W alloy is used as the alloy harder than the bar cap 1.