To provide a method for checking connection of the connector pin in a current sensor having a current detection coil wound around an opening/ closing magnetic core and connected, at the opposite ends thereof, with pins at specified position of an output connector through a two-core cord easily using a simple tester.

One line of a two line power supply cord for feeding a tester 3 with an AC operating power is enclosed, in same direction, by a reference current sensor 1 having correct pin connection and a current sensor 2 to be tested and detected currents are fed through connectors P1, J1 and P2, J2 to the tester 3. The tester 3 converts the input currents into voltages which are then added and compared with a specified reference voltage by means of a comparator 7. When the added voltage exceeds the reference voltage, the comparator 7 delivers an L level (or H level) signal for driving a display circuit to light an LED therein thus informing the connection of the connector pin in the current sensor 2 to be tested is acceptable.