To provide a method for forming openings economically.

When forming openings, longitudinal sections are cut by wire saws at the right and left of the scheduled opening positions thereby creating the longitudinal section lines. Next, core boring holes 20 are formed in the places for scheduled openings subdivided by the longitudinal section lines. The core boreholes 20 have both horizontal core boreholes 20a and vertical core boreholes 20b. Next, within the first step horizontal core boreholes 20a, cylinder heads 24 are respectively inserted and installed, these are enlarged and operated at the same time and the lateral cutting line B1 is formed by its breaking. Next, a cylinder head 24 is inserted and installed in a vertical core borehole 20b provided at the upper portion side from the first step horizontal borehole 20a, and enlarging operation is performed and longitudinal cutting line C1 is formed. When the longitudinal cutting line C1 is formed, the portion at the upper portion side from the first step horizontal borehole 20a is fully separated at the portion of the longitudinal cutting line C1 and divided into a block 34. The divided block 34 is sequentially removed.