An elastic contact piece is formed from a front edge of a bottom portion forming a box-shaped body part. A contact portion extended from a side edge of the elastic contact piece faces a window hole formed on a side wall. In a stopper forming a lower edge of the window hole, a receiving portion is formed on a cut-off portion in such a manner that a lengthwise center portion of the cut-off portion tapers off inward. Because the contact portion is received by the receiving portion of the stopper, the elastic contact piece is prevented from deforming excessively. At this time, the contact portion is received in a wide range as far as a deep portion thereof by the receiving portion tapering off inward. Thus, the force acting on the contact portion can be dispersed.