PURPOSE:To sensitively detect the contact of a tool without fitting a special detector to a work by monitoring the oscillation frequency or oscillation output of an ultrasonic oscillator and discriminating the variation amt. thereof exceeding a specified threshold value. CONSTITUTION:A tool contact detecting device 3 is connected to the ultrasonic oscillator 22 exciting an ultrasonic vibrator 25, the output detecting means 1 thereof detects the oscillation frequency or oscillation output of the ultrasonic oscillation 22 and a variation amt. discriminating means 2 discriminates the variation amt. thereof exceeding a specified threshold value. The oscillation frequency or oscillation output is read in order simultaneously with feeding started by the command of NC device and the variation amt. from the initial state is calculated. Whether or not exceeding the specified threshold value is discriminated successively, if not exceeding, a tool 27 is under approaching and when the exceeding is discriminated, the coordinate at that time is read by NC device, a feeding motor is stopped and the position thereof is that where the tool 27 is brought into contact with a work W.