An O-ring 3 used in a clamp-type joint 10 for vacuum apparatus, which engages with a concave groove 21 in the outer circumference edge of the center ring 2 and is sandwiched by an outer ring 5 having a U-shaped outer circumference, characterized in that the diameter of the circular arc in the cross-section on the side coming into contact with the outer ring 5 is smaller than the diameter of the circular arc in the cross-section on the axial side of the joint. A clamp-type joint for vacuum apparatus which can exhibit stable sealing performance at a high temperature of 200° C. or more without cracking of the O-ring, as well as an outer ring and O-ring used with the clamp-type joint are provided.