To prevent molten metal from flowing in at the time of casting by making a lid closable at a socket side end surface of a metal frame of a divided core composed of a reusable metal without preventing diameter enlarging and reducing movements of a mold.

An inner mold 10 is mounted in order to mold the socket inner surface at the side end of the socket 4 of the metal frame 1 which centrifugally casts a tube 8 having a socket 9, and has a flange portion 11 abutted at the side end of the socket 4 of the metal frame 1, a core portion 12 coaxially extending backward into the socket on the inner surface of the tube socket and a divided core portion 13 further extending backward into the socket of the tube socket. Divided portions 13a, 13b comprising the divided core portion 13 are supported by each flange portion 11 to allow the diameter to be shortened and enlarged and a lid body 15 is provided to close the leading end of the divided core 13 in the tip end surface of the divided core portion 13.