A golf ball comprising a core and a cover positioned outside the core, wherein the core includes a center and an envelope layer positioned outside the center, the center is formed by a rubber composition being crosslinked, the envelope layer is formed by a rubber composition being crosslinked, at least one of the rubber composition of the center and the rubber composition of the envelope layer includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, wherein the acid and/or salt (d) excludes the co-crosslinking agent (b), and wherein the acid and/or salt (d) is selected from the group consisting of carboxylic acids, sulfonic acids, and phosphoric acid, hydroacids, and salts thereof, the co-crosslinking agent (b) is: an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, the JIS-C hardness H(Ie) at an innermost portion of the envelope layer is greater than a JIS-C hardness H(Sc) at an outermost portion of the center, the JIS-C hardness Ho of the cover is greater than a JIS-C hardness Hs at a surface of the core, and if the JIS-C hardnesses are measured at 11 measuring points from the core center to the core surface and the distances from the central point of the core to each of these measuring points are plotted in a graph, then at the envelope layer R 2 of the linear approximation curve obtained by the least-square method is equal to or greater than 0.93. the core includes a center and an envelope layer positioned outside the center, the center is formed by a rubber composition being crosslinked, the envelope layer is formed by a rubber composition being crosslinked, at least one of the rubber composition of the center and the rubber composition of the envelope layer includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, wherein the acid and/or salt (d) excludes the co-crosslinking agent (b), and wherein the acid and/or salt (d) is selected from the group consisting of carboxylic acids, sulfonic acids, and phosphoric acid, hydroacids, and salts thereof, (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, wherein the acid and/or salt (d) excludes the co-crosslinking agent (b), and wherein the acid and/or salt (d) is selected from the group consisting of carboxylic acids, sulfonic acids, and phosphoric acid, hydroacids, and salts thereof, the co-crosslinking agent (b) is: an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, the JIS-C hardness H(Ie) at an innermost portion of the envelope layer is greater than a JIS-C hardness H(Sc) at an outermost portion of the center, the JIS-C hardness Ho of the cover is greater than a JIS-C hardness Hs at a surface of the core, and if the JIS-C hardnesses are measured at 11 measuring points from the core center to the core surface and the distances from the central point of the core to each of these measuring points are plotted in a graph, then at the envelope layer R 2 of the linear approximation curve obtained by the least-square method is equal to or greater than 0.93.