PROBLEM TO BE SOLVED: To extend a blasting length and thereby to improve the speed of execution of work by making the amount of overbreak smaller than usual by contriving disposition of blast holes. SOLUTION: In this method, blast holes 3 having a depth of 1/2L (half of a usual one) are bored almost at the same positions as those of prior-art blast holes 5 in a cutting face 2 of an underground cavity 1 and, moreover, blast holes 4 having a depth L are bored on the side inward by about d2 from the blast holes 3. By blasting explosives charged in these blast holes 3 and 4, the underground cavity 1 is excavated by utilizing an SB effect. Although the amount of excavation per one blast is L, almost the same as the usual one, accordingly, the blast holes 3 and 4 lie deep radially only by d2=1/2×d1, i.e., half of a usual depth, from a design side face 1a and, therefore, the amount of overbreak is reduced to half of the usual one.