Abstract: In a spark plug 1 of the present invention, in a mirror-polished surface 230a obtained by mirror-polishing a cut surface 230 obtained by cutting an insulator 2 in a direction perpendicular to an axial line AX direction, at a position separated by 2 mm from a portion having a maximum diameter of a diameter-enlarged portion 31a to a rear end side along the axial line AX direction, when 20 first observation regions X each being 192 ?m×255 ?m are set so as to each overlap a reference position m1 being a position 0.2 mm in a radial direction from an inner peripheral surface 2a side of the insulator 2 and so as not to overlap each other, a proportion (porosity) of an area of all of pores included in the 20 first observation regions X relative to a total area (100%) of the 20 first observation regions X is not greater than 3.

Abstract: A spark plug 1 of the present invention includes: an insulator 2 made from an alumina-based sintered body; a center electrode being a bar-like electrode inserted in the insulator 2 and having, on a rear end side of the center electrode, a diameter-enlarged portion enlarged in a radial direction and engaged with an inner wall of the insulator; and a conductive sealing material provided on the rear end side of the center electrode 3 inside the insulator.

Abstract: A spark plug 1 of the present invention includes: an insulator 2 made from an alumina-based sintered body; a center electrode being a bar-like electrode inserted in the insulator 2 and having, on a rear end side of the center electrode, a diameter-enlarged portion enlarged in a radial direction and engaged with an inner wall of the insulator; and a conductive sealing material provided on the rear end side of the center electrode 3 inside the insulator.

Abstract: In a spark plug 1 of the present invention, in a mirror-polished surface 230a obtained by mirror-polishing a cut surface 230 obtained by cutting an insulator 2 in a direction perpendicular to an axial line AX direction, at a position separated by 2 mm from a portion having a maximum diameter of a diameter-enlarged portion 31a to a rear end side along the axial line AX direction, when 20 first observation regions X each being 192 ?m×255 ?m are set so as to each overlap a reference position ml being a position 0.2 mm in a radial direction from an inner peripheral surface 2a side of the insulator 2 and so as not to overlap each other, a proportion (porosity) of an area of all of pores included in the 20 first observation regions X relative to a total area (100%) of the 20 first observation regions X is not greater than 3.