A spark plug, comprising: a center electrode; a ceramic insulator having an axial hole extending in an axial direction and retaining the center electrode in the axial hole; a metal shell circumferentially surrounding and retaining therein the ceramic insulator; a ground electrode joined at one end portion thereof to a front end face of the metal shell and bent in such a manner that a side surface of the other end portion (31,) of the ground electrode faces a front end portion of the center electrode; and a spark section joined to the side surface of the other end portion of the ground electrode at a position facing the front end portion of the center electrode, the spark section comprising: an intermediate member containing nickel as a main component, joined to the side surface and protruding toward the center electrode; and a noble metal member containing a noble metal as a main component and joined to a protruding end of the intermediate member so as to define a spark gap between the noble metal member and the front end portion of the center electrode, wherein a thermal conductivity of the intermediate member is lower than a thermal conductivity of the noble metal member charactrized in that the spark section protrudes 0.6 to 1.6 mm from the side surface toward the center electrode, a portion of the noble metal member adjacent to a fused joint between the noble metal member and the intermediate member has an outer diameter Dp of 0.5 to 1.2 mm, the spark section satisfies a relationship of -0.1 mm ≤ Dn-Dp ≤ 0.5 mm where Dn is an outer diameter of the intermediate member before being joined. a center electrode; a ceramic insulator having an axial hole extending in an axial direction and retaining the center electrode in the axial hole; a metal shell circumferentially surrounding and retaining therein the ceramic insulator; a ground electrode joined at one end portion thereof to a front end face of the metal shell and bent in such a manner that a side surface of the other end portion (31,) of the ground electrode faces a front end portion of the center electrode; and a spark section joined to the side surface of the other end portion of the ground electrode at a position facing the front end portion of the center electrode, the spark section comprising: an intermediate member containing nickel as a main component, joined to the side surface and protruding toward the center electrode; and a noble metal member containing a noble metal as a main component and joined to a protruding end of the intermediate member so as to define a spark gap between the noble metal member and the front end portion of the center electrode, wherein a thermal conductivity of the intermediate member is lower than a thermal conductivity of the noble metal member charactrized in that the spark section protrudes 0.6 to 1.6 mm from the side surface toward the center electrode, a portion of the noble metal member adjacent to a fused joint between the noble metal member and the intermediate member has an outer diameter Dp of 0.5 to 1.2 mm, the spark section satisfies a relationship of -0.1 mm ≤ Dn-Dp ≤ 0.5 mm where Dn is an outer diameter of the intermediate member before being joined. an intermediate member containing nickel as a main component, joined to the side surface and protruding toward the center electrode; and a noble metal member containing a noble metal as a main component and joined to a protruding end of the intermediate member so as to define a spark gap between the noble metal member and the front end portion of the center electrode, wherein a thermal conductivity of the intermediate member is lower than a thermal conductivity of the noble metal member charactrized in that the spark section protrudes 0.6 to 1.6 mm from the side surface toward the center electrode, a portion of the noble metal member adjacent to a fused joint between the noble metal member and the intermediate member has an outer diameter Dp of 0.5 to 1.2 mm, the spark section satisfies a relationship of -0.1 mm ≤ Dn-Dp ≤ 0.5 mm where Dn is an outer diameter of the intermediate member before being joined.