Spark plug

A predetermined cross section (55) of a noble metal tip (50) is divided by a vertical line (L) into two half cross sections, which are a first half cross section (60) and a second half cross section (70) different from the first half cross section (60). The first half cross section (60) satisfies Expressions 1 and 2, and the second half cross section (70) satisfies Expressions 3 and 4. h 1 / t ‰¤ 0.2 , Rw 1 / Rt 1 ‰¥ 1.03 h 2 / t ‰¤ 0.2 , Rw 2 / Rt 2 ‰¥ 1.03 At a welding interface (80), oxidation scale progresses in a "direction away from an axis OL" along side surfaces (63, 73), and changes the progressing direction at the end points (65, 75) so as to progress in a "direction toward the axis OL" along bottom surfaces (62, 72). Therefore, progress of oxidation scale is restrained. Also, since the noble metal tip (50) is held by an engagement portion (45) of a ground electrode (40), separation of the noble metal tip (50) from the ground electrode (40) can be restrained.