PURPOSE: To simplify the apparatus so as to allow the testing of a printed circuit board by a visual sense of a testing person, by displaying a pattern on a liquid crystal display panel corresponding to a potential distribution on a solder surface of a printed circuit board to be tested.

CONSTITUTION: A power source 8 is connected to a printed circuit board 3 to be tested and an oscillator 2 to the printed circuit board 3 being tested and a transparent electrode 5 and when a solder surface of the printed circuit board 3 being tested in operation is brought into contact with an insulation thin film 7 of a liquid crystal display plate 1, an electric field is applied to a liquid crystal 6 closed according to potential at a contact part. Consequently, optical refractive index of the liquid crystal 6 varies with parts and a pattern appears on a liquid crystal display surface 1 through an optical filter 4 according to a potential distribution on the solder surface of the printed circuit board 3 being tested. Thus, the state of the potential distribution on the solder surface of the printed circuit board 3 being tested appears on the liquid crystal display plate 1 as pattern thereby simplifying the apparatus with easier testing by a visual sense.