A spark plug, comprising: a tubular metal shell extending along an axis; and a solid annular gasket disposed at an outer periphery of the metal shell, wherein the metal shell includes: a thread portion for mounting formed at an outer periphery on a tip end side of the metal shell; a seat portion formed on a rear end side with respect to the thread portion, the seat portion bulging radially outward; and a thread root portion disposed between the thread portion and the seat portion, wherein the gasket has an inner diameter smaller than a thread size of the thread portion, the gasket being disposed at an outer periphery of the thread root portion and in contact with the seating portion, characterized in that the following formulas are satisfied: 0.8L ≤ C ≤ L - 2A and B ≤ 0.75L; wherein L (mm) is an outer diameter of a seating, portion, which seating portion is a surface of the seat portion positioned at the gasket side; A (mm) is a maximum value of a relatively shiftable distance of the gasket along a direction orthogonal to the axis relative to the metal shell from a state where the axis and a central axis of the gasket coincide with each other; C (mm) is an outer diameter of a face of the gasket in contact with the seating portion; and B (mm) is an inner diameter of a face of the gasket in contact with the seating portion. a tubular metal shell extending along an axis; and a solid annular gasket disposed at an outer periphery of the metal shell, wherein the metal shell includes: a thread portion for mounting formed at an outer periphery on a tip end side of the metal shell; a seat portion formed on a rear end side with respect to the thread portion, the seat portion bulging radially outward; and a thread root portion disposed between the thread portion and the seat portion, wherein a thread portion for mounting formed at an outer periphery on a tip end side of the metal shell; a seat portion formed on a rear end side with respect to the thread portion, the seat portion bulging radially outward; and a thread root portion disposed between the thread portion and the seat portion, wherein the gasket has an inner diameter smaller than a thread size of the thread portion, the gasket being disposed at an outer periphery of the thread root portion and in contact with the seating portion, characterized in that the following formulas are satisfied: 0.8L ≤ C ≤ L - 2A and B ≤ 0.75L; wherein L (mm) is an outer diameter of a seating, portion, which seating portion is a surface of the seat portion positioned at the gasket side; A (mm) is a maximum value of a relatively shiftable distance of the gasket along a direction orthogonal to the axis relative to the metal shell from a state where the axis and a central axis of the gasket coincide with each other; C (mm) is an outer diameter of a face of the gasket in contact with the seating portion; and B (mm) is an inner diameter of a face of the gasket in contact with the seating portion. 0.8L ≤ C ≤ L - 2A and B ≤ 0.75L; wherein L (mm) is an outer diameter of a seating, portion, which seating portion is a surface of the seat portion positioned at the gasket side; A (mm) is a maximum value of a relatively shiftable distance of the gasket along a direction orthogonal to the axis relative to the metal shell from a state where the axis and a central axis of the gasket coincide with each other; C (mm) is an outer diameter of a face of the gasket in contact with the seating portion; and B (mm) is an inner diameter of a face of the gasket in contact with the seating portion.