A spark plug (100, 100a, 100b, 100c), comprising: a center electrode that extends in an axial direction (D1, D2); an insulator that has an axial hole extending in the axial direction (D1, D2), the center electrode being to be inserted into the axial hole; a metal shell (50, 50b, 50c) arranged at an outer periphery of the insulator; a first ground electrode (30, 30a, 30b) that has electrical continuity with the metal shell (50, 50b, 50c), the first ground electrode forming a first gap with a front end surface (28s1) of the center electrode; and a second ground electrode (90, 90a, 90b, 90c) that has electrical continuity with the metal shell (50, 50b, 50c), the second ground electrode being sealed to the metal shell (50, 50b, 50c), the second ground electrode extending from the metal shell (50, 50b, 50c) to a position facing a side surface (28s2) of the center electrode, the second ground electrode forming an annular second gap between the side surface (28s2) of the center electrode and an inner peripheral surface of the second ground electrode, characterized in that a proportion of a size of the first gap to a size of the second gap is equal to or more than 0.80 and equal to or less than 1.25; and in that at least one of the first ground electrode (30a, 30b) and the second ground electrode (90a, 90b, 90c) includes: a surface layer (36, 36b, 96, 96b, 96c) that forms a surface thereof; and a core portion (37, 37b, 97) that is formed inside of the surface layer (36, 36b, 96, 96b, 96c) and has a larger thermal conductivity than a thermal conductivity of the surface layer; and and in that the first ground electrode is sealed to the second ground electrode (90b, 90c). a center electrode that extends in an axial direction (D1, D2); an insulator that has an axial hole extending in the axial direction (D1, D2), the center electrode being to be inserted into the axial hole; a metal shell (50, 50b, 50c) arranged at an outer periphery of the insulator; a first ground electrode (30, 30a, 30b) that has electrical continuity with the metal shell (50, 50b, 50c), the first ground electrode forming a first gap with a front end surface (28s1) of the center electrode; and a second ground electrode (90, 90a, 90b, 90c) that has electrical continuity with the metal shell (50, 50b, 50c), the second ground electrode being sealed to the metal shell (50, 50b, 50c), the second ground electrode extending from the metal shell (50, 50b, 50c) to a position facing a side surface (28s2) of the center electrode, the second ground electrode forming an annular second gap between the side surface (28s2) of the center electrode and an inner peripheral surface of the second ground electrode, characterized in that a proportion of a size of the first gap to a size of the second gap is equal to or more than 0.80 and equal to or less than 1.25; and in that at least one of the first ground electrode (30a, 30b) and the second ground electrode (90a, 90b, 90c) includes: a surface layer (36, 36b, 96, 96b, 96c) that forms a surface thereof; and a core portion (37, 37b, 97) that is formed inside of the surface layer (36, 36b, 96, 96b, 96c) and has a larger thermal conductivity than a thermal conductivity of the surface layer; and and in that the first ground electrode is sealed to the second ground electrode (90b, 90c).