PURPOSE: To discover the disconnected point of the conductor circuit section of a printed board from a difference in brightness in an obtained picture by observing the conductor circuit section under a scanning electron microscope while a potential is applied to the conductor circuit section.

CONSTITUTION: A printed board 3 is provided with a first and second conductor circuit sections 1 and 2. While both circuit sections 1 and 2 are originally connected to each other, the sections 1 and 2 have a disconnected section 6 for testing. Then a voltage is applied between the sections 1 and 2 by means of a power supply circuit 5, namely, the anode and cathode of the battery incorporated in the circuit 5 are respectively connected to the sections 1 and 2. Under such conducting condition, the picture of the sections 1 and 2 is taken by irradiating the sections 1 and 2 with an electron beam 4 from a scanning electron microscope. In the picture, the section 1 is shown dark, whereas the section 2 is shown white. Therefore, the boundary point at which the difference in brightness is observed can be discriminated as a disconnected point.