PURPOSE: To permit the number of revolution set beforehand to be always maintained irrespectively of load variation of a spindle by constituting the captioned apparatus form a speed setting device for setting the revolution speed of a drilling machine to an arbitrary value and a speed governor in which the power source voltage of a motor can be varied automatically.

CONSTITUTION: A drilling machine consists of an electrical oscillator 1, cycle setting device 2, drilling machine 3, spindle 4, pulse converter 5 etc., and said cycle setting device 2 sets the cycle corresponding to the necessary revolution number, on the basis of the original cycle of the electrical oscillator 1. A silicone control element 9 feeds a driving electric power for keeping the number of revolution of the drilling machine 3 to a constant value, and possesses the function to keep the revolution number of the spindle 4, at the number of revolution set by the cycle setting device 2, even if the load of the spindle 4 varies.