A plug which does not cause an adverse effect such as failure of a member constituting the plug when a wellbore is plugged. The downhole plug (10) includes a mandrel (1) having a hollow structure, a sealing member (2) attached to an outer circumferential surface of the mandrel (1) and being deformable by exerted pressure, and a socket (3) attached, adjacent to the sealing member (2), onto the outer circumferential surface of the mandrel (1), downstream of where the pressure is exerted on the sealing member (2). An inner circumferential edge (31a) of the socket (3) which comes in contact with the mandrel (1) protrudes toward the sealing member (2).