A spark plug comprising: a tubular insulator extending in an axial direction, parallel to an axial line, from a rear end side toward a forward end side, the insulator having a step portion having a surface facing the forward end side; a tubular metallic shell for holding the insulator thereinside, the metallic shell including a ledge that supports the step portion and a middle hole portion located on the rear end side of the ledge and connected to the ledge, the ledge having an inner surface facing the rear end side; and a sheet packing held between the step portion and the ledge; the spark plug being characterized in that: a thickness of the sheet packing at a midpoint (P9a, P9b) of the inner surface in the cross section is 0.15 mm or more and 0.20 mm or less; and in that 2.8 ≤ (A + B)/M holds, where A (mm) is a sum of a length A1 (mm) and a length A2 (mm), the length A1 being a length of contact between the sheet packing and the metallic shell in one of two half sections obtained by dividing, by the axial line, a cross section of the spark plug that passes through the axial line, the length A2 being a length of contact between the sheet packing and the insulator in the one of the two half sections, B (mm) is a sum of a length B1 (mm) and a length B2 (mm), the length B1 being a length of contact between the sheet packing and the metallic shell in the other one of the two half sections that is different from the one of the two half sections, the length B2 being a length of contact between the sheet packing and the insulator in the other one of the two half sections, and M (mm) is a difference obtained by subtracting an inner diameter D (mm) of the ledge from an inner diameter C (mm) of the middle hole portion. a tubular insulator extending in an axial direction, parallel to an axial line, from a rear end side toward a forward end side, the insulator having a step portion having a surface facing the forward end side; a tubular metallic shell for holding the insulator thereinside, the metallic shell including a ledge that supports the step portion and a middle hole portion located on the rear end side of the ledge and connected to the ledge, the ledge having an inner surface facing the rear end side; and a sheet packing held between the step portion and the ledge; the spark plug being characterized in that: a thickness of the sheet packing at a midpoint (P9a, P9b) of the inner surface in the cross section is 0.15 mm or more and 0.20 mm or less; and in that 2.8 ≤ (A + B)/M holds, where A (mm) is a sum of a length A1 (mm) and a length A2 (mm), the length A1 being a length of contact between the sheet packing and the metallic shell in one of two half sections obtained by dividing, by the axial line, a cross section of the spark plug that passes through the axial line, the length A2 being a length of contact between the sheet packing and the insulator in the one of the two half sections, B (mm) is a sum of a length B1 (mm) and a length B2 (mm), the length B1 being a length of contact between the sheet packing and the metallic shell in the other one of the two half sections that is different from the one of the two half sections, the length B2 being a length of contact between the sheet packing and the insulator in the other one of the two half sections, and M (mm) is a difference obtained by subtracting an inner diameter D (mm) of the ledge from an inner diameter C (mm) of the middle hole portion. a thickness of the sheet packing at a midpoint (P9a, P9b) of the inner surface in the cross section is 0.15 mm or more and 0.20 mm or less; and in that 2.8 ≤ (A + B)/M holds, where A (mm) is a sum of a length A1 (mm) and a length A2 (mm), the length A1 being a length of contact between the sheet packing and the metallic shell in one of two half sections obtained by dividing, by the axial line, a cross section of the spark plug that passes through the axial line, the length A2 being a length of contact between the sheet packing and the insulator in the one of the two half sections, B (mm) is a sum of a length B1 (mm) and a length B2 (mm), the length B1 being a length of contact between the sheet packing and the metallic shell in the other one of the two half sections that is different from the one of the two half sections, the length B2 being a length of contact between the sheet packing and the insulator in the other one of the two half sections, and M (mm) is a difference obtained by subtracting an inner diameter D (mm) of the ledge from an inner diameter C (mm) of the middle hole portion.