A retaining device includes a first member, a second member, a third member, a support section supporting the first member and supporting the second member and the third member movably relative to the first member, and a manipulation part moving the first member, the second member, and the third member relative to each other. The first member and the third member can grasp a first tissue with the first tissue disposed between the first member and the second member and between the first member and the third member, and the first member and the second member can grasp the first tissue and a second tissue such that the second tissue is disposed between the first member and the second member and between the second member and the third member to contact with the first tissue with the first member and the third member grasping the first tissue.
