Spark plug

Abstract

In a spark plugof the present invention, in a mirror-polished surfaceobtained by mirror-polishing a cut surfaceobtained by cutting an insulatorin 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 portionto a rear end side along the axial line AX direction, when 20 first observation regions X each being 192 μm×255 μm are set so as to each overlap a reference position mbeing a position 0.2 mm in a radial direction from an inner peripheral surfaceside of the insulatorand 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 3.5%, and in a thermally etched surfaceobtained by subjecting the mirror-polished surfaceto thermal etching, when 20 second observation regions Y each being 32 μm×43 μm are set so as to each overlap the reference position mand so as not to overlap each other, a particle size distribution of alumina particles included in the 20 second observation regions Y is regarded as a normal distribution, an average particle diameter of the alumina particles is defined as A, and a standard deviation of a particle diameter of the alumina particles is defined as σ, A is not less than 1.9 μm and not greater than 2.8 μm, and (A+3σ) is not greater than 3.0 μm.