A spark plug (100/200/300/400/500) comprising: a center electrode; an insulator having an axial hole extending in an axial direction of the center electrode, and holding the center electrode in a front end portion of the axial hole; a metallic shell having a cylindrical hole extending in the axial direction, and having, within the cylindrical hole, a holding portion being in direct or indirect contact with an outer surface of the insulator along a whole circumference of the insulator and adapted to hold the insulator in the cylindrical hole; a ground electrode whose one end portion is joined to the metallic shell and which is bent such that another end portion thereof faces a front end portion of the center electrode and defines a spark discharge gap in cooperation with the front end portion of the center electrode; an isolation portion defined as a portion of the insulator located, in the axial direction, frontward of a position Q where, as viewed from a front side in the axial direction, the insulator first comes in direct or indirect contact with the holding portion; wherein a greatest outside diameter of the isolation portion of the insulator is equal to or less than an outside diameter of the insulator as measured at the position Q; characterized in that a portion of the outer surface of the isolation portion which faces an inwardly oriented surface which partially constitutes the holding portion and faces inward with respect to a radial direction orthogonal to the axial direction is disposed such that a gap of 0.4 mm or less in the radial direction is formed between the portion and the inwardly oriented surface along a whole circumference of the portion, and a ratio (S/V) of a surface area S of the outer surface of the isolation portion of the insulator to a volume V of the isolation portion of the insulator satisfies a relation 1.26 mm -1 ≤ S/V ≤ 1.40 mm -1. a center electrode; an insulator having an axial hole extending in an axial direction of the center electrode, and holding the center electrode in a front end portion of the axial hole; a metallic shell having a cylindrical hole extending in the axial direction, and having, within the cylindrical hole, a holding portion being in direct or indirect contact with an outer surface of the insulator along a whole circumference of the insulator and adapted to hold the insulator in the cylindrical hole; a ground electrode whose one end portion is joined to the metallic shell and which is bent such that another end portion thereof faces a front end portion of the center electrode and defines a spark discharge gap in cooperation with the front end portion of the center electrode; an isolation portion defined as a portion of the insulator located, in the axial direction, frontward of a position Q where, as viewed from a front side in the axial direction, the insulator first comes in direct or indirect contact with the holding portion; wherein a greatest outside diameter of the isolation portion of the insulator is equal to or less than an outside diameter of the insulator as measured at the position Q; characterized in that a portion of the outer surface of the isolation portion which faces an inwardly oriented surface which partially constitutes the holding portion and faces inward with respect to a radial direction orthogonal to the axial direction is disposed such that a gap of 0.4 mm or less in the radial direction is formed between the portion and the inwardly oriented surface along a whole circumference of the portion, and a ratio (S/V) of a surface area S of the outer surface of the isolation portion of the insulator to a volume V of the isolation portion of the insulator satisfies a relation 1.26 mm -1 ≤ S/V ≤ 1.40 mm -1.