To automatically color the surface of an object without the necessity to perform a complicated operation by arranging a contact sensor which has a contact element coming into contact with the surface of the object and moves relatively to the object in the three-dimensional direction to output a signal showing the contact state of the contact element.

A contact element 60 is formed of a columnar stainless steel having a specified strength and is fixed to the front side of a lower edge part 52a of a metal plate 52 using a fixture 56. When voltage is applied to a space between the metal plate 52 of a contact sensor 50 and a main electrode 72b through a lead wire 80 and a lead wire 82, a piezoelectric element 54 is distorted in proportion to voltage under a piezoelectric effect, which results in the oscillation of the metal plate 52 due to the distortion of the piezoelectric element 54. Thus the contact element 60 arranged on the metal plate 52 also begins to oscillate. The contact sensor 50 is fixedly arranged on a mounting member 32 through a spacer made up of a tubular body with a threaded hole formed on the inner peripheral side, near the aperture part, of both end parts of the spacer.