Piston ring and piston ring set

A piston ring comprising an annular main body portion having an inner peripheral surface, an outer peripheral surface, and a first side surface and a second side surface substantially orthogonal to the inner peripheral surface, the outer peripheral surface of the piston ring comprising:

• a first curved surface provided on a side of the main body portion toward the first side surface, the first curved surface being convexly curved outwards in a radial direction of the main body portion;
• a second curved surface provided on a side of the main body portion toward the second side surface, the second curved surface being convexly curved outwards in the radial direction of the main body portion;
• and
• an intermediate surface formed to extend in a width direction of the main body portion so as to connect the first curved surface to the second curved surface, wherein
• the intermediate surface has a flat portion, and
• a width of the intermediate surface in the width direction is between 0.05 mm and 0.30 mm.