The present invention relates to method and apparatus for drilling a large diameter hole in the ground. The present invention comprises a hammer drill (40) installed at the bottom of a rod (104) assembled in multi-stages at support means on the ground comprising a pilot hammer drill (44, 48, 49) and a reamer hammer drill (41, 46, 47) coupled to each other through a key structure (42, 43) formed in the vertical direction so as to allow simultaneous rotation and up-and-down movement relative to each other, wherein in a drilling operation, a repetition is made such that the hammer drill (40) is made to rotate and the pilot hammer drill (44, 48, 49) is lowered to drill a pilot hole of small diameter (w) to a predetermined depth (d) and is lifted up to original position, and then the reamer hammer drill (41, 46, 47) is lowered to enlarge the hole to a large diameter (W) down to the predetermined depth (d). Accordingly, the present invention provides effects that a hole of extra large diameter which has not been possible to drill using a single conventional hammer can be drilled, and a hole of large diameter can be drilled using a relatively small drilling machine in drilling operations, thereby reducing the time required to drill holes, facilitating the drilling operations and maintaining a higher level of hole verticality.