SPARK PLUG

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The present invention addresses the problem of providing a spark plug with which generation of cracks starting at the boundaries of oxidized grains in an outer layer can be suppressed in a cooling/heating environment while temperature elevations of an earth electrode are suppressed. The spark plug according to the present invention comprises a central electrode and an earth electrode having a gap with respect to the central electrode, the earth electrode having at least a core and an outer layer that surrounds the core, and the core being formed of a material having higher thermal conductivity than the outer layer, wherein the spark plug is characterized in that there is at least a site where the thickness of the outer layer at a cross section orthogonal to the direction in which the earth electrode extends is 0.5 mm or less, and the composition of an electrode material forming the outer layer includes 96 mass% or more of Ni, 0.05 mass% or more for the total of at least one selected from the group comprising Y and rare earth elements, 0.5 mass% or less of Al, and 0.5 to 1.5 mass% of Si (where the total of Ni, Y, rare earth elements, Al and Si does not exceed 100 mass%).