A plug for well drilling comprising: a mandrel; at least one diametrically expandable annular rubber member; and at least one slip; the at least one diametically expandable circular rubber member and the at least one slip being disposed on an outer circumferential surface orthogonal to an axial direction of the mandrel; (i) the mandrel being formed from a degradable material; characterized in that (ii) the at least one diametrically expandable annular rubber member being formed from a degradable rubber material has: A rate of decrease of not less than 5 % in 50 % strain compressive stress, as measured in accordance with JIS K7181 after immersion in water at a temperature of 150°C for 24 hours relative to a 50 % strain compressive stress before immersion; a tensile fracture strain, as measured in accordance with JIS K6251, of not less than 50 % at a temperature of 66°C; a 70 % strain compressive stress, as measured in accordance with JIS K7018, of not less than 10 MPa at a temperature of 66°C; and a compressive fracture strain, as measured in accordance with JIS K7018, of not less than 50 % at a temperature of 66°C, the degradable rubber material being an ester-based urethane rubber. a mandrel; at least one diametrically expandable annular rubber member; and at least one slip; the at least one diametically expandable circular rubber member and the at least one slip being disposed on an outer circumferential surface orthogonal to an axial direction of the mandrel; (i) the mandrel being formed from a degradable material; characterized in that (ii) the at least one diametrically expandable annular rubber member being formed from a degradable rubber material has: A rate of decrease of not less than 5 % in 50 % strain compressive stress, as measured in accordance with JIS K7181 after immersion in water at a temperature of 150°C for 24 hours relative to a 50 % strain compressive stress before immersion; a tensile fracture strain, as measured in accordance with JIS K6251, of not less than 50 % at a temperature of 66°C; a 70 % strain compressive stress, as measured in accordance with JIS K7018, of not less than 10 MPa at a temperature of 66°C; and a compressive fracture strain, as measured in accordance with JIS K7018, of not less than 50 % at a temperature of 66°C, the degradable rubber material being an ester-based urethane rubber. (i) the mandrel being formed from a degradable material; characterized in that (ii) the at least one diametrically expandable annular rubber member being formed from a degradable rubber material has: A rate of decrease of not less than 5 % in 50 % strain compressive stress, as measured in accordance with JIS K7181 after immersion in water at a temperature of 150°C for 24 hours relative to a 50 % strain compressive stress before immersion; a tensile fracture strain, as measured in accordance with JIS K6251, of not less than 50 % at a temperature of 66°C; a 70 % strain compressive stress, as measured in accordance with JIS K7018, of not less than 10 MPa at a temperature of 66°C; and a compressive fracture strain, as measured in accordance with JIS K7018, of not less than 50 % at a temperature of 66°C, the degradable rubber material being an ester-based urethane rubber. A rate of decrease of not less than 5 % in 50 % strain compressive stress, as measured in accordance with JIS K7181 after immersion in water at a temperature of 150°C for 24 hours relative to a 50 % strain compressive stress before immersion; a tensile fracture strain, as measured in accordance with JIS K6251, of not less than 50 % at a temperature of 66°C; a 70 % strain compressive stress, as measured in accordance with JIS K7018, of not less than 10 MPa at a temperature of 66°C; and a compressive fracture strain, as measured in accordance with JIS K7018, of not less than 50 % at a temperature of 66°C, the degradable rubber material being an ester-based urethane rubber.