The present invention includes a lower plate, an upper plate provided above the lower plate in such a manner that the upper plate is biased so as to be vertically movable, a guide member attached to the upper plate, the guide member guiding vertical movement of the upper plate, and a spring provided below the guide member, the spring biasing the upper plate upward via the guide member. In the lower plate, an insertion hole having a size that allows the spring and the guide member to pass therethrough in a vertical direction is formed. In the insertion hole, a spring holding member that holds the spring is detachably provided. Such configuration enables suppression of warpage of the upper plate caused by a biasing force of the spring and enables work for replacement of the upper plate to be performed easily and quickly.