A member coupling mechanism capable of coupling two members using a necessary tightening force even in a limited space by a small and simple configuration. A member coupling mechanism includes a first member including a member coupling portion having an open apical surface, a second member coupled to the member coupling portion of the first member, an elastic sealing member inserted between the first member and the second member to seal an opening of the apical surface of the member coupling portion, a coupling member that couples the first member and the second member together, and an acting portion that exerts a force on the coupling member.