A first golf ball 2 includes a core 4, a mid-layer 6, and a cover 8. The core 4 includes a center 10 and an envelope layer 12. The mid-layer 6 includes a metal ion source (C) that can neutralize an ionomer resin. The mid-layer 6 has a melt flow rate (at 190° C. and 2.16 kg) of 4 g/10 minute or greater. The difference (H3−H1) between the JIS-C hardness H3 of the cover 8 and the JIS-C central hardness H1 of the center 10 is equal to or greater than 45. A second golf ball 22 includes a core 24, a mid-layer 28, and a cover 30. The mid-layer 28 has a melt flow rate (at 190° C. and 2.16 kg) of 4 g/10 minute or greater. Preferably, the mid-layer 28 includes a high melt viscosity resin (A), a low melt viscosity ionomer resin (B), and a metal ion source (C1).