To readily cope with even a kelly-bar having diverse sizes and manufactured by different makers by providing an adapter that can drive a kelly-bar having diverse sizes such as an outer diameter or a rib by one driving unit.

The adaptor 9 has a driven rib 10 capable of being engaged with a driving rib provided in the inside of a turn table 6 in an earth drill, and the adapter 9 has in the inside thereof a driving rib 16 capable of being engaged with the driven rib 10 provided outside of a kelly-bar having a different size, and also has a stopper 17 for putting the adapter 9 on a rotary table 6 at the upper part of the adapter 9 and a disconnection prevention clamping ring 12 for preventing the adapter 9 from being disconnected thereof.