PURPOSE: To aim at improvements in both of operatability and workability, by adding a drill diameter detector and an optimum revolution controller to an electric drill, while making this drill selectable automatically for automatic detection for a drill diameter and optimum revolution according to the drill diameter and a work material.

CONSTITUTION: An electric drill comprises a drill diameter detector and an optimum revolution controller 29. Now, before a trigger switch is turned to ON, if data of a work material is inputted into a microcomputer 24 by means of an input device 23, the microcomputer 24 outputs a display data commensurate to the input data to a work material display device 221. Simultaneously, the microcomputer 24 inputs a drill diameter data out of a potentiometer 14 through an A/D converter 27 and calculates the drill diameter, outputting the calculated data to a drill diameter revolution display device 222. And, the computer 24 selects the data of the optimum revolution commensurate to the work material and the drill diameter from a read-only memory, and outputs it to a circuit 29. With this constitution, operatability and workability both are improvable.