24 and the inclined surface 32 is divided into the one inclined surface 32 a of a rising gradient which extends toward the one edge 33 and the other inclined surface 32 b of a rising gradient which extends toward the other edge 34 by contrast as in a modification shown in fig1 the rising inclined surface 32 which is directed toward the upper edge of the stepped surface 31 may be formed by a single flat surface in the configuration of fig1 in which the inclined surface 32 is divided into the one inclined surface 32 a and the other inclined surface 32 b as compared with that of fig1 in which the inclined surface 32 is formed by the single flat surface the rising gradient of the other inclined surface 32 b is larger than that of the inclined surface 32 of fig1 and hence the movement resistance on the other inclined surface 32 b which is exerted when the engagement end 41 moves along the escape path 26 to the return path start portion 23 a is large therefore the locked state of the slider 70 is hardly cancelled by a reaction due to a drop impact or the like as in the configuration of fig1 in which the other edge.