diameter reducing die member 4 there is assembled a circular cylindrical inside reinforcement ring 5 around which there is assembled a circular cylindrical outside reinforcement ring 6 further on the underside of the die member 4 and reinforcement rings 5 6 there is disposed a pressure receiving plate 7 on the underside of which there is disposed a cap 9 the cap 9 is fastened to a circular cylindrical case 8 by means of bolts 10 the case 8 covers the outer peripheral surfaces of the reinforcement rings 3 6 and pressure receiving plate 7 by the cap 9 being fastened to the case 8 the die members 2 and 4 rings 3 5 and 6 and the like are integrated together and in this condition these are disposed on a bolster not illustrated of a presser the diameter reducing die member 4 is made of silicon nitride the guiding die member 2 and the reinforcement rings 3 5 and 6 are each made of skd 61 which is one of the alloy tool steels for use in hot working for example the die member 4 may be made of another ceramic material such as sialon also the die member 2 and the reinforcement rings 3 5 and 6 may each be made of another alloy tool steel for use in hot working or the like such as skd 6 further the guiding die member 2 may be made of silicon nitride or ceramic material.