A spark plug, comprising: an insulator having a through hole formed in the direction of an axis of the spark plug; a rod-shaped center electrode inserted in the through hole and extending in the direction of the axis; a metal shell disposed around an outer circumference of the insulator; and a ground electrode electrically connected with the metal shell and adapted to define a gap between the ground electrode and the center electrode, wherein a front end part of the insulator has a front end surface, an outer circumferential surface extending toward the rear from the front end surface in the direction of the axis and a curved surface region formed between the front end surface and the outer circumferential surface; characterized in that: a front end of the center electrode protrudes frontward from the insulator; in a cross section including the axis, a front end of an inner circumferential surface of the metal shell faces the curved surface region in a direction perpendicular to the axis; the curved surface region has a curvature radius of 0.2 mm to 0.8 mm; and a condition 0 ≤ ΔH ≤ H1 is fulfilled, wherein ΔH represents the position of the front end part of the curved surface region of the ceramic insulator with respect to the position of the front end of the inner circumferential surface of the metal shell in the axial direction, and H1 represents a length of the curved surface region in the axial direction. an insulator having a through hole formed in the direction of an axis of the spark plug; a rod-shaped center electrode inserted in the through hole and extending in the direction of the axis; a metal shell disposed around an outer circumference of the insulator; and a ground electrode electrically connected with the metal shell and adapted to define a gap between the ground electrode and the center electrode, wherein a front end part of the insulator has a front end surface, an outer circumferential surface extending toward the rear from the front end surface in the direction of the axis and a curved surface region formed between the front end surface and the outer circumferential surface; characterized in that: a front end of the center electrode protrudes frontward from the insulator; in a cross section including the axis, a front end of an inner circumferential surface of the metal shell faces the curved surface region in a direction perpendicular to the axis; the curved surface region has a curvature radius of 0.2 mm to 0.8 mm; and a condition 0 ≤ ΔH ≤ H1 is fulfilled, wherein ΔH represents the position of the front end part of the curved surface region of the ceramic insulator with respect to the position of the front end of the inner circumferential surface of the metal shell in the axial direction, and H1 represents a length of the curved surface region in the axial direction. a front end of the center electrode protrudes frontward from the insulator; in a cross section including the axis, a front end of an inner circumferential surface of the metal shell faces the curved surface region in a direction perpendicular to the axis; the curved surface region has a curvature radius of 0.2 mm to 0.8 mm; and a condition 0 ≤ ΔH ≤ H1 is fulfilled, wherein ΔH represents the position of the front end part of the curved surface region of the ceramic insulator with respect to the position of the front end of the inner circumferential surface of the metal shell in the axial direction, and H1 represents a length of the curved surface region in the axial direction.