A spark plug comprising: a rod-like center electrode; an insulator assuming a cylindrical form and having therein a bore extending in an axial direction, the insulator accommodating the center electrode in a front end of the bore; a metal shell assuming a cylindrical form, accommodating and holding therein the insulator with a stepped portion formed on an inner circumference thereof for engaging with a support portion formed on an outer circumference of the insulator; and an annular packing fitted in an intervening manner between the support portion on the outer circumference of the insulator and the stepped portion on the inner circumference of the metal shell, wherein, in a cross-section including an axial line of the spark plug, the following relationship is satisfied, 0.6 mm < = L, where "A" represents a connection point between the support portion of the insulator and an insulator trunk portion formed at a front end side with respect to the support portion of the insulator, where "B" represents a position closer to the outer circumference side along the surface of the support portion of the insulator among positions of (a) an innermost position of a contact portion where the support portion of the insulator and the packing are in contact with each other and (b) an intersection of the support portion of the insulator and a virtual straight line that is parallel to the axial line and extends from an innermost circumferential end of the stepped portion of the metal shell, and where "L" represents a length of a path from the point "A" to the point "B" along a surface of the insulator, characterized in that the following relationship is satisfied: r 1 − r 2 < = 0.5 ⁢ mm, where "r1" represents a radius of an inner circumference of a metal shell shelf positioned frontwards with respect to the stepped portion of the metal shell, and where "r2" represents a radius of an outer circumference of a portion that faces a front end of the metal shell shelf in the insulator trunk portion. a rod-like center electrode; an insulator assuming a cylindrical form and having therein a bore extending in an axial direction, the insulator accommodating the center electrode in a front end of the bore; a metal shell assuming a cylindrical form, accommodating and holding therein the insulator with a stepped portion formed on an inner circumference thereof for engaging with a support portion formed on an outer circumference of the insulator; and an annular packing fitted in an intervening manner between the support portion on the outer circumference of the insulator and the stepped portion on the inner circumference of the metal shell, wherein, in a cross-section including an axial line of the spark plug, the following relationship is satisfied, 0.6 mm < = L, where "A" represents a connection point between the support portion of the insulator and an insulator trunk portion formed at a front end side with respect to the support portion of the insulator, where "B" represents a position closer to the outer circumference side along the surface of the support portion of the insulator among positions of (a) an innermost position of a contact portion where the support portion of the insulator and the packing are in contact with each other and (b) an intersection of the support portion of the insulator and a virtual straight line that is parallel to the axial line and extends from an innermost circumferential end of the stepped portion of the metal shell, and where "L" represents a length of a path from the point "A" to the point "B" along a surface of the insulator, characterized in that the following relationship is satisfied: r 1 − r 2 < = 0.5 ⁢ mm, where "r1" represents a radius of an inner circumference of a metal shell shelf positioned frontwards with respect to the stepped portion of the metal shell, and where "r2" represents a radius of an outer circumference of a portion that faces a front end of the metal shell shelf in the insulator trunk portion.