A generation of a stress corrosion cracking is prevented in a middle section. The spark plug 1 includes an insulator 2 that extends along an axis CL1 direction and a metal shell 3. The metal shell 3 includes a middle section 41 that is positioned between a collar section 16 and a tool engaging section 19. The middle section 41 has a bulged section 42 that bulges towards both the inside and the outside in the diametrical direction, a first slender section 43 that is the most slender at a portion that is positioned further to the rear end side in the axis CL1 dircetion than the bulged section 42 and a second slender section 44 that is the most slender at a portion that is positioned further to the leading end side in the axis CL1 direction than the bulged section 42 and the bulged section 42 has the most bulged section 42M that bulges toward the most inside of diametrical direction. In a cross-section that includes the axis CL1, when the distance between both slender sections 43 and 44 along the axis CL1 is F (mm) and the bulged amount to the inside in diametrical direction of the most bulged section 42M with respect to an imaginary line that connects a portion that is positioned furthest to the inside in diametrical direction of both slender sections 43 and 44 is G (mm), 0.00