An engagement structure includes a cover having an opening portion, a block member inserted into the opening portion, an engagement mechanism configured to engage the block member with the opening portion, and an engagement stabilization mechanism configured to stabilize an engagement state of the engagement mechanism. The engagement stabilization mechanism includes a first stabilization mechanism and a second stabilization mechanism. The first stabilization mechanism permits excessive insertion due to backlash that occurs in the engagement mechanism and to restore the excessive insertion after the block member is engaged with the opening portion. The second stabilization mechanism performs a slide engagement between the block member and the opening portion and to restrict a wall inner surface of the cover from moving away from an outer surface of a side portion of the block member after the slide engagement is completed.