A method of mechanized construction has developed for installing an earth wire under-bracing by using an auger crane and adjustable excavating unit with an auger crane. The extendable excavating unit is mounted to the lower end of an auger shaft of the auger crane and excavation teeth extendable and retractable are mounted to the excavating unit, thereby allowing excavation of a burial hole suitable to the size of the earth wire under-bracing. Whether it is needed to bury the earth wire under-bracing having a larger diameter than the diameter of excavation existing on the auger crane, or bury small-sized earth wire under-bracing, a method for mechanized construction of the earth wire under-bracing is enabled without difficulty by using an auger crane and adjustable excavating unit.