Described herein is a method and equipment for making an impermeable diaphragm of secant piles, based upon the execution of parallel holes with constant distances between centres, said method and equipment being of the type that uses a drill rod (5) equipped, at the bottom end, with a bit (16) for making adjacent holes (1, 1', 2) of substantially circular cross section, in which the equipment is constituted by a guide (6, 7), constrained to the drill rod (5), and made up of two elements shaped so as to press against the walls of two adjacent holes (1, 2); the two elements are connected to one another through a slider (20) passing through a vertical opening (21) made between the two holes (1, 2).