PURPOSE: To improve the detection of capacitance the vertical direction between the nozzle tip and a work by transforming the change of capacitance into the change of frequency and controlling the distance between the nozzle tip and the work based on the counted value.

CONSTITUTION: An oscillator 2 shifts phase of a signal from a sensor electrode 1a, and changes and amplifies the frequency. A signal in accordance with the amplified frequency is then set and fed back to a shielding electrode 1b. As a result, a counter 3 capable of finely changing the signal to the shielding electrode 1b according to the outside environment where working is carried out for the capacitance detected from the distance between the nozzle tip and the work is constituted of a frequency counter and generates a pulse based on the phase of the frequency-modulated signal by the oscillator 2. This pulse is then counted for every specified period and outputted to a control part 4. As a result, excessive noises can be eliminated and an accurate digital value in accordance with the distance between the nozzle tip and the work can be detected.