A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein the core comprises a center and an envelope layer positioned outside the center, the center is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, the co-crosslinking agent (b) is: (b1) an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, a Shore D hardness Hm of the mid layer is greater than a Shore D hardness Hc of the cover, wherein the co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d), and when distances (%) from a central point of the core to five points and JIS-C hardnesses at the five points are plotted in a graph, wherein the five points are obtained from the central point of the core to a surface of the core, wherein the ratio of the distance from the central point of the core to each of these measuring points to the radius of the core is: first point at 0.0% (central point), second point at 12.5%, third point at 25.0%, fourth point at 37.5% and fifth point at 45.0%, R 2 of a linear approximation curve obtained by a least-square method at the center is equal to or greater than 0.95, wherein the difference (Hm-Hc) between the hardness Hm and the hardness Hc is equal to or greater than 27, wherein an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a). the core comprises a center and an envelope layer positioned outside the center, the center is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, the co-crosslinking agent (b) is: (b1) an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, (b1) an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms, a Shore D hardness Hm of the mid layer is greater than a Shore D hardness Hc of the cover, wherein the co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d), and when distances (%) from a central point of the core to five points and JIS-C hardnesses at the five points are plotted in a graph, wherein the five points are obtained from the central point of the core to a surface of the core, wherein the ratio of the distance from the central point of the core to each of these measuring points to the radius of the core is: first point at 0.0% (central point), second point at 12.5%, third point at 25.0%, fourth point at 37.5% and fifth point at 45.0%, R 2 of a linear approximation curve obtained by a least-square method at the center is equal to or greater than 0.95, wherein the difference (Hm-Hc) between the hardness Hm and the hardness Hc is equal to or greater than 27, wherein an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a). <EOS>