PURPOSE: To find surely a defective conduction, by detecting the connection state between a connecting terminal and a coated electric wire on a basis of the voltage drop which is generated when a small current is flowed between the connecting terminal and the coated electric wire to make them conductive.

CONSTITUTION: Electrodes 8 and 8' are surely brought into contact with one ends of press-fixing terminals 6 and 6', which are press-fixed to a coated electric wire 1 by a press-fixing machine to be surely held, and a small current corresponding to the diameter of a core wire part 1a of the electric wire 1 is flowed. Since there is a fear that a current is flowed across the part 1a and terminals 6 and 6' even in case of an incomplete connection if a large current is flowed, thus small current is flowed. The small current of a constant voltage is flowed by a power supply device 11; and if terminals 6 and 6' are press-fixed surely to the conductor part 1a of the electric wire 1, a prescribed voltage drop is generated, and the voltage drop at this time is detected by a voltmeter 9.