PURPOSE: To confirm that the connection is made surely as to connectors of various forms, by providing two or more signal lines and one earth line, and confirming the connecting condition of the connector connection as a surface.

CONSTITUTION: When a male connector 2 and a female connector 3 are not connected, signal lines 8a and 8b are connected to a power source 101 to make the input side of a logical circuit 7 in a high potential, and the output of the logical circuit 7 is made fasle. When the connector 2 and the connector 3 are connected obliquely, the signal line 8a is connected to a short-circuit signal line 9a to make into a closed circuit, so as to make into zero potential by the earth 102, but the signal line 8b is not made to a closed circuit but to a high potential, so as to make the logical OR output false. When the connector 2 and the connector 3 are connected regularly, the signal lines 8a and 8b are short-circuited with the earth 102, and the input side of the logical circuit 7 is all made into zero potential, so as to make the logical OR output true. And, these outputs are displayed on the connection display 10. Consequently, the connecting condition can be confirmed surely.