Brackets 9, 9 for supporting respective opposite ends of a guide
rod 4 are provided with respective cam faces 95, 95 approximately
symmetric about an approximate longitudinal midportion C of the guide rod 4,
and at least one of the brackets 9 is provided with an adjusting member
for moving the guide rod 4 longitudinally thereof. The cam faces 9 guide
the guide rod 4 toward a direction in which the guide rod 4 pivotally
moves about the approximate longitudinal midportion C, as the guide rod 4 is
moved longitudinally thereof by the adjusting member.
