Spark plug and spark plug for the metal shell

An adhesion of a nickel plating layer to a metal shell is improved, and an effect of improving a corrosion resistance by providing the nickel plating layer is sufficiently exerted. A spark plug 1 includes a cylindrical metal shell 3 that extends in an axial CL1 direction, and a nickel plating layer 31 that is made of metal containing nickel as a main component and covers an outer surface of the metal shell. In a monochrome image of 256 gradations where black is 0 and white is 255, which is obtained by observing a cross-section perpendicular to an outer surface of the nickel plating layer by a transmission electron microscope with an acceleration voltage of 200 kV, an average value in 256 gradations of the monochrome image is 170 or larger and 230 or smaller.