A dissecting device for dissecting tissue surrounding a blood vessel in a living body. The dissecting device includes a grasping section that includes an insertion lumen. The dissecting device includes a dissecting member at the distal portion of the grasping section. The dissecting member is insertable into the living body along the blood vessel to dissect the tissue surrounding the blood vessel in the living body. The dissecting member includes a treating section and a protruding section. The treating section performs a predetermined treatment of a branch vessel branching from the blood vessel. The protruding section protrudes from the treating section in the thickness direction of the treating section. The protruding section includes a guide section configured to guide the branch vessel toward the treating section.
