PURPOSE: To remove a knock-out piece easily even in the case that the knock- out piece is moved to the direction opposite to a connection part and is fitted in a knock-out hole.

CONSTITUTION: A knock-out piece 12 is punched in a substrate 2, leaving a connection part 11, to form a knock-out hole 13. A groove hole 14 is opened between the outer edge of the knock-out piece 12 and the inner edge of the knock-out hole 13 in a nearly half circumferential area θ opposite to the connection part 11.