A core 4 of a golf ball 2 includes a center 10 and an envelope layer 12 positioned outside the center 10. The ratio of the volume of the core 4 to the volume of a phantom sphere of the golf ball 2 is equal to or greater than 78%. The difference (He−Ho) between a JIS-C hardness He at the surface of the core 4 and a JIS-C hardness Ho at the central point of the core 4 is equal to or greater than 15 but equal to or less than 40. The difference (H1−Ho) between a JIS-C hardness H1 at a point P1 that is located radially outward of the boundary between the center 10 and the envelope layer 12 and whose distance from the boundary is 1 mm and the JIS-C hardness Ho is equal to or greater than 0 but equal to or less than 2.