A spark plug, comprising: a center electrode extending in an axial line direction (Df, Dfr); an insulator that includes an axial hole extending in the axial line direction (Df, Dfr), the center electrode being arranged at a front end side (Df) of the axial hole, the insulator including an outer-diameter-contracted portion and a nose portion, the outer-diameter-contracted portion having an outer diameter decreasing toward the front end side (Df) in the axial line direction (Df, Dfr), the nose portion being a part disposed at a front end side (Df) of the outer-diameter-contracted portion; a metal shell arranged at an outer periphery of the insulator, the metal shell including an inner-diameter-contracted portion, the inner-diameter-contracted portion having an internal diameter decreasing toward the front end side (Df) in the axial line direction (Df, Dfr); and a packing arranged between the outer-diameter-contracted portion of the insulator and the inner-diameter-contracted portion of the metal shell, wherein in a contact portion of the packing and the insulator, a position at a most front end side (Df) is set as a first position (Pa), in a surface of the nose portion of the insulator, a position where a length from a front end (10e1) of the insulator parallel to the axial line direction (Df, Dfr) is 1 mm is set as a second position (Pb), a length between the first position (Pa) and the second position (Pb) parallel to the axial line direction (Df, Dfr) is set as a first length, characterized in that the spark plug is configured such that in a case where a load perpendicular to the axial line direction (Df, Dfr) is applied to the second position (Pb) in a state where the insulator is secured at the first position (Pa) of the insulator and the front end (10e1) of the insulator is a free end, a ratio of stress (Sti) at a surface position (Pi) that is a position within the range from the first position (Pa) to the second position (Pb) on a surface of the insulator to stress (Sta) at the first position (Pa) is set as a stress ratio (Sti/Sta), and in a range of the surface position where the stress ratio (Sti/Sta) is 0.8 or more to 1.15 or less, a length in a continuous range from the first position (Pa) toward a front end (10e1) side parallel to the axial line direction (Df, Dfr) is set as a second length, a ratio (L2/L1) of the second length to the first length is 0.7 or more; and wherein Sti = Mi / Zi, and Sta = Ma / Za, wherein Mi is the moment at the surface position (Pi); Ma is the moment at the first position; Zi is the section modulus at the surface position (Pi); and Za is the section modulus at the first position; and Sti is stress at the surface position (Pi); and Sta is stress at the first position. a center electrode extending in an axial line direction (Df, Dfr); an insulator that includes an axial hole extending in the axial line direction (Df, Dfr), the center electrode being arranged at a front end side (Df) of the axial hole, the insulator including an outer-diameter-contracted portion and a nose portion, the outer-diameter-contracted portion having an outer diameter decreasing toward the front end side (Df) in the axial line direction (Df, Dfr), the nose portion being a part disposed at a front end side (Df) of the outer-diameter-contracted portion; a metal shell arranged at an outer periphery of the insulator, the metal shell including an inner-diameter-contracted portion, the inner-diameter-contracted portion having an internal diameter decreasing toward the front end side (Df) in the axial line direction (Df, Dfr); and a packing arranged between the outer-diameter-contracted portion of the insulator and the inner-diameter-contracted portion of the metal shell, wherein in a contact portion of the packing and the insulator, a position at a most front end side (Df) is set as a first position (Pa), in a surface of the nose portion of the insulator, a position where a length from a front end (10e1) of the insulator parallel to the axial line direction (Df, Dfr) is 1 mm is set as a second position (Pb), a length between the first position (Pa) and the second position (Pb) parallel to the axial line direction (Df, Dfr) is set as a first length, characterized in that the spark plug is configured such that in a case where a load perpendicular to the axial line direction (Df, Dfr) is applied to the second position (Pb) in a state where the insulator is secured at the first position (Pa) of the insulator and the front end (10e1) of the insulator is a free end, a ratio of stress (Sti) at a surface position (Pi) that is a position within the range from the first position (Pa) to the second position (Pb) on a surface of the insulator to stress (Sta) at the first position (Pa) is set as a stress ratio (Sti/Sta), and in a range of the surface position where the stress ratio (Sti/Sta) is 0.8 or more to 1.15 or less, a length in a continuous range from the first position (Pa) toward a front end (10e1) side parallel to the axial line direction (Df, Dfr) is set as a second length, a ratio (L2/L1) of the second length to the first length is 0.7 or more; and wherein Sti = Mi / Zi, and Sta = Ma / Za, wherein Mi is the moment at the surface position (Pi); Ma is the moment at the first position; Zi is the section modulus at the surface position (Pi); and Za is the section modulus at the first position; and Sti is stress at the surface position (Pi); and Sta is stress at the first position.