A spark plug for an internal combustion engine comprising: a cylindrical insulating body that extends in an axis direction; and a cylindrical metal shell that is fixed on an outer periphery of the insulating body, wherein the metal shell includes: a collar section that bulges toward an outside of a diametrical direction, a tool engaging section to which a tool is engaged to attach the metal shell to the internal combustion engine, and a middle section that is positioned between the collar section and the tool engaging section, wherein the middle section has a bulged section that bulges toward both an inside and the outside in the diametrical direction, wherein the middle section has a first slender section that is a portion positioned at further rear end side in the axis direction than the bulged section and is a section that is the most slender in the portion, and a second slender section that is a portion positioned at further leading end side in the axis direction than the bulged section of the middle section and is a section that is the most slender in the portion, wherein the bulged section has a most bulged section that is a portion that bulges furthest to the inside in the diametrical direction, characterized in that in a cross-section including the axis, assuming that F (mm) is a distance between the first slender section and the second slender section along the axis, and G (mm) is a bulged amount toward the inside in diametrical direction of the most bulged section with respect to an imaginary line that connects a portion that is positioned furthest to the inside in the diametrical direction of the first slender section and a portion that is positioned furthest to the inside in the diametrical direction of the second slender section, following formula is satisfied 0.00 < G / F ≤ 0.18 a cylindrical insulating body that extends in an axis direction; and a cylindrical metal shell that is fixed on an outer periphery of the insulating body, wherein the metal shell includes: a collar section that bulges toward an outside of a diametrical direction, a tool engaging section to which a tool is engaged to attach the metal shell to the internal combustion engine, and a middle section that is positioned between the collar section and the tool engaging section, a collar section that bulges toward an outside of a diametrical direction, a tool engaging section to which a tool is engaged to attach the metal shell to the internal combustion engine, and a middle section that is positioned between the collar section and the tool engaging section, wherein the middle section has a bulged section that bulges toward both an inside and the outside in the diametrical direction, wherein the middle section has a first slender section that is a portion positioned at further rear end side in the axis direction than the bulged section and is a section that is the most slender in the portion, and a second slender section that is a portion positioned at further leading end side in the axis direction than the bulged section of the middle section and is a section that is the most slender in the portion, wherein the bulged section has a most bulged section that is a portion that bulges furthest to the inside in the diametrical direction, characterized in that in a cross-section including the axis, assuming that F (mm) is a distance between the first slender section and the second slender section along the axis, and G (mm) is a bulged amount toward the inside in diametrical direction of the most bulged section with respect to an imaginary line that connects a portion that is positioned furthest to the inside in the diametrical direction of the first slender section and a portion that is positioned furthest to the inside in the diametrical direction of the second slender section, following formula is satisfied 0.00 < G / F ≤ 0.18