A downhole plug (10) includes a mandrel (1) having a hollow structure, an annular sealing member (2) attached to an outer circumferential surface of the mandrel (1) and being deformable by exerted pressure, and an annular socket (3) attached, adjacent to the annular sealing member (2), onto the outer circumferential surface of the mandrel (1), downstream of where a pressure is exerted on the sealing member (2). The annular socket (3) is configured to include a socket-inner portion (31) which comes into contact with the outer circumferential surface of the mandrel (1), and a socket-outer portion (32) in an annular shape having an inner diameter equivalent to or larger than that of the socket-inner portion (31) and movably attached to the socket-inner portion (31). The socket-outer portion (32) includes a face facing the sealing member (2).