a state in which one end thereof passes through a hole formed in the case main body 106 a and the other end thereof passes through a hole formed in the lid 106 b a flange 107 a is formed on a portion on the lid 106 b side of the plunger 107 the falling off of the plunger 107 from the spring case 106 is restricted by the plunger 107 a the coil spring 103 is interposed between the bottom of the case 106 a and the flange 107 a the plunger 107 is biased toward the lid 106 b side by the repulsive force of the coil spring 103 the spring case 106 is positioned such that the upper end of the plunger 107 faces to the underside of the dowel 8 b formed on the frame 8 a of the seat 8 in other words the seat 8 is pushed upwardly by the plunger 107 biased by the coil spring 103 the seating switch 54 is fixed on the side surface of the spring case 106 the spindle 54 b passes through holes formed in the flange 106 c of the case main body 106 a and the lid 106 b and projects upwardly therefrom the seating switch 54 is positioned such that the leading end of the spindle 54 b is not brought into contact with the underside of the dowel 8 b when the seat 8 is upwardly pushed to the uppermost position by the plunger 107 a state in which the dowel 8 b is engaged with the link member 100 is shown in fig1 b fig1 b is a sectional view taken on line x u2014 x of fig1 a as shown in this figure a projection t is.