A spark plug, comprising: a center electrode; a ceramic insulator having an axial hole extending in an axial direction and retaining the center electrode in the axial hole; a metal shell surrounding a radial outer circumference of the ceramic insulator and retaining therein the ceramic insulator; a ground electrode joined at one end thereof to a front end face of the metal shell and having a bent portion formed between the one end and the other end thereof in such a manner that the other end of the ground electrode faces a front end of the center electrode; and an electrode tip joined to the other end of the ground electrode at a position facing the front end of the center electrode and having a protrusion length of 0.5 mm or larger from the other end of the ground electrode and a cross sectional area of 0.20 to 1.13 mm 2, the ground electrode having a layered structure consisting of three to five layers formed by structural members (35, 36, 37) laminated together, said structural elements including a first structural member extending from the one end toward the other end of the ground electrode, a second structural member laminated on an outer surface of the first structural member, and a third structural member laminated on an outer surface of the second structural member, the minimum curvature radius of a side surface of the bent portion facing the center electrode being 2.3 mm or smaller; the length of protrusion of a point of the other end of the ground electrode protruding most in the axial direction from the front end face of the metal shell being 4.5 mm or larger; the metal shell having a mounting thread formed with a nominal diameter of M12 or smaller based on JIS standard; and the total thermal conductivity X of the ground electrode as expressed by the formula being 35 W/(m·K) or higher at 20°C [Formula 4] X = v o l u m e o f f i r s t s t r u c t u r a l member v o l u m e o f g r o u n d electrode × t h e r m a l c o n d u c t i v i t y o f f i r s t s t r u c t u r a l member + ∑ i = 2 n v o l u m e o f i t h s t r u c t u r a l member v o l u m e o f g r o u n d electrode × t h e r m a l c o n d u c t i v i t y o f i t h s t r u c t u r a l member where n is an integer of 3 to 5 indicating the maximum number of the structural members (35, 36, 37) of the ground electrode. a center electrode; a ceramic insulator having an axial hole extending in an axial direction and retaining the center electrode in the axial hole; a metal shell surrounding a radial outer circumference of the ceramic insulator and retaining therein the ceramic insulator; a ground electrode joined at one end thereof to a front end face of the metal shell and having a bent portion formed between the one end and the other end thereof in such a manner that the other end of the ground electrode faces a front end of the center electrode; and an electrode tip joined to the other end of the ground electrode at a position facing the front end of the center electrode and having a protrusion length of 0.5 mm or larger from the other end of the ground electrode and a cross sectional area of 0.20 to 1.13 mm 2, the ground electrode having a layered structure consisting of three to five layers formed by structural members (35, 36, 37) laminated together, said structural elements including a first structural member extending from the one end toward the other end of the ground electrode, a second structural member laminated on an outer surface of the first structural member, and a third structural member laminated on an outer surface of the second structural member, the minimum curvature radius of a side surface of the bent portion facing the center electrode being 2.3 mm or smaller; the length of protrusion of a point of the other end of the ground electrode protruding most in the axial direction from the front end face of the metal shell being 4.5 mm or larger; the metal shell having a mounting thread formed with a nominal diameter of M12 or smaller based on JIS standard; and the total thermal conductivity X of the ground electrode as expressed by the formula being 35 W/(m·K) or higher at 20°C [Formula 4] X = v o l u m e o f f i r s t s t r u c t u r a l member v o l u m e o f g r o u n d electrode × t h e r m a l c o n d u c t i v i t y o f f i r s t s t r u c t u r a l member + ∑ i = 2 n v o l u m e o f i t h s t r u c t u r a l member v o l u m e o f g r o u n d electrode × t h e r m a l c o n d u c t i v i t y o f i t h s t r u c t u r a l member where n is an integer of 3 to 5 indicating the maximum number of the structural members (35, 36, 37) of the ground electrode.