A pressing portion is housed movably in a front-and-back direction inside a main member portion. In the main member portion, two main-member-portion holes through which a string member passes are formed on a side wall forming an opening portion. The pressing portion includes a pressing face on a front face, and internally provides a pressing-portion string insertion portion which can communicate with the two main-member-portion holes and through which the string member passes. A bend insertion portion for inserting the string member to pass through by bending the string member inside the pressing portion is provided. An urging device for urging the pressing portion is provided at a position wrapping a virtual three-dimensional shape obtained by connecting, with straight lines, two imaginary faces on which an inner face of the side wall and an outer shape of the string member passing through the two main-member-portion holes intersect.