ca and cu in composition ratios 1 8 0 4 2 2 2 3 this powder was charged in a silver pipe of 4 mm in inner diameter and 6 mm in outer diameter drawn into a wire of 1 6 mm in diameter subjected to cold rolling to be 0 2 mm in thickness and subjected to first heat treatment under respective conditions shown in table 4 thereafter respective samples v 1 to v 8 were subjected to cold rolling to be 0 14 to 0 17 mm in thickness and subjected to second heat treatment at 845 u00b0 c for 50 hours table 4 shows characteristics of the respective samples thus obtained particularly critical current density values under dipping in liquid nitrogen high critical current density values were obtained in the samples v 5 to v 8 which were subjected to first heat treatment under conditions for generating high temperature phases i e conditions of at least u201c 0845 u00b0 c for 20 hours u201d example of a fifth embodiment of the present invention is now described commercially available powder materials 99 9 to 99 99 in purity of bi 2 o 3 pbo sb 2 o 3 srco 3 caco 3 and cuo of about 1 u03bcm in average particle size were mixed in composition ratios shown in table 5 such samples were calcined at 800 u00b0 c for 12 hours pulverized sintered at 800 u00b0 c for 18 hours and pulverized and then charged in silver pipes of 12 mm.