A hairspring comprising: a main body portion that extends around a central axis along an Archimedes curve; an outer end curve portion that extends around the central axis on an outer side in a radial direction from the main body portion; and a reformed portion that connects the main body portion and the outer end curve portion to each other includes a first bent portion that is bent outward in the radial direction from a tangent line of the Archimedes curve in a first connection portion with the main body portion, and a second bent portion bent inward in the radial direction from the outer end curve portion in a second connection portion with the outer end curve portion, and in which both of the first bent portion and the second bent portion are curved, characterized in that, where an inner diameter of the outermost circumferential portion of an original shape of the hairspring is defined as a time diameter on a virtual straight line that passes through the central axis and the outer end portion of the original shape of the hairspring when viewed from the shaft direction, the time diameter satisfies 3.5 mm ≤ D ≤ 5.5 mm, the radius of curvature of the outer end curve portion satisfies 0.25 mm ≤ R-(D/2) ≤ 0.65 mm, and a center angle (θ) on the central axis between both ends of the reformed portion satisfies 10° ≤ θ ≤ 35°, the radius of curvature of the first bent portion satisfies 3 mm ≤ R1 ≤ 7 mm and the radius of curvature of the second bent portion satisfies 0.5 mm ≤ R2 ≤ 1 mm, the original shape of the hairspring being the shape of the hairspring when not deformed by the reformed portion with the entire hairspring having a shape that extends along the Archimedes curve. a main body portion that extends around a central axis along an Archimedes curve; an outer end curve portion that extends around the central axis on an outer side in a radial direction from the main body portion; and a reformed portion that connects the main body portion and the outer end curve portion to each other includes a first bent portion that is bent outward in the radial direction from a tangent line of the Archimedes curve in a first connection portion with the main body portion, and a second bent portion bent inward in the radial direction from the outer end curve portion in a second connection portion with the outer end curve portion, and in which both of the first bent portion and the second bent portion are curved, characterized in that, where an inner diameter of the outermost circumferential portion of an original shape of the hairspring is defined as a time diameter on a virtual straight line that passes through the central axis and the outer end portion of the original shape of the hairspring when viewed from the shaft direction, the time diameter satisfies 3.5 mm ≤ D ≤ 5.5 mm, the radius of curvature of the outer end curve portion satisfies 0.25 mm ≤ R-(D/2) ≤ 0.65 mm, and a center angle (θ) on the central axis between both ends of the reformed portion satisfies 10° ≤ θ ≤ 35°, the radius of curvature of the first bent portion satisfies 3 mm ≤ R1 ≤ 7 mm and the radius of curvature of the second bent portion satisfies 0.5 mm ≤ R2 ≤ 1 mm, the original shape of the hairspring being the shape of the hairspring when not deformed by the reformed portion with the entire hairspring having a shape that extends along the Archimedes curve.