A spark plug for an internal combustion engine, comprising: a center electrode extending in the direction of an axis of the spark plug and having a core of higher thermal expansion coefficient than that of a front end thereof, the center electrode including a flanged portion radially outwardly protruding on a rear side thereof and a cylindrical portion located closer to a front end of the spark plug than the flanged portion and being smaller in diameter than the flanged portion; an insulator having an axial hole formed in the direction of the axis to retain the flanged portion in the axial hole with the cylindrical portion held in a loose-fit state in the axial hole; and a metal shell accommodating the insulator; wherein there is a plurality of regions of substantially constant diameter difference between the inner diameter of the axial hole and the outer diameter of the cylindrical portion, including a first constant diameter difference region that is the longest in the direction of the axis; wherein an axial position B is located in the first constant diameter difference region; and wherein an axial position F is located closer to the front end of the spark plug than the first constant diameter difference region; wherein the spark plug satisfies the following condition: Cb < Cf where Cb is a diameter difference between an inner diameter of the axial hole and an outer diameter of the cylindrical portion at the axial position B, and Cf is a diameter difference between the inner diameter of the axial hole and the outer diameter of the cylindrical portion at the axial position F; wherein Cb is set to 0.01 mm to 0.09 mm; and wherein the spark plug satisfies the following condition: 0 < Cf - Cb <= 0.06 mm. a center electrode extending in the direction of an axis of the spark plug and having a core of higher thermal expansion coefficient than that of a front end thereof, the center electrode including a flanged portion radially outwardly protruding on a rear side thereof and a cylindrical portion located closer to a front end of the spark plug than the flanged portion and being smaller in diameter than the flanged portion; an insulator having an axial hole formed in the direction of the axis to retain the flanged portion in the axial hole with the cylindrical portion held in a loose-fit state in the axial hole; and a metal shell accommodating the insulator; wherein there is a plurality of regions of substantially constant diameter difference between the inner diameter of the axial hole and the outer diameter of the cylindrical portion, including a first constant diameter difference region that is the longest in the direction of the axis; wherein an axial position B is located in the first constant diameter difference region; and wherein an axial position F is located closer to the front end of the spark plug than the first constant diameter difference region; wherein the spark plug satisfies the following condition: Cb < Cf where Cb is a diameter difference between an inner diameter of the axial hole and an outer diameter of the cylindrical portion at the axial position B, and Cf is a diameter difference between the inner diameter of the axial hole and the outer diameter of the cylindrical portion at the axial position F; wherein Cb is set to 0.01 mm to 0.09 mm; and wherein the spark plug satisfies the following condition: 0 < Cf - Cb <= 0.06 mm.