PURPOSE: To enable easy replacement of only the inside die while a lower die is held set in a press b attaching freely attachably and detachably the inside die to the lower die body via a fixing pin by means of a ring for detaining the fixing pin.

CONSTITUTION: A lower die body 10 of a rotary forging machine is fixed to a rotary table 3 by means of a lock bolt 4 and an approximately cylindrical hollow part C is formed in the position near a center of rotation 11 to reduce the weight of a lower die A. An annular mounting groove 14 is formed on the center 11 side of the caliber 12 of the body 10 and an inside die 13 is mounted therein. The die 13 is adequately selected in accordance with the bore size, etc. of a product B. A lock pin 15 is fitted into a through-hole 16 communicating with the 13 and the body 10. Said pin 15 is detained by an approximately cylindrical detaining ring 17. The easy detachment of the die 13 is thus made possible by removing the ring 17 from the hollow part C of the body 10 and taking out the pin 15.