Spark plug with an improved separation resistance of a noble metal tip

A spark plug where a predetermined cross section of a noble metal tip is divided by a vertical line into two half cross sections having a first half cross section and a second half cross section. The first half cross section satisfies h1/t 0.2 and Rw1/Rt1 1.03, and the second half cross section satisfies h2/t 0.2 and Rw2/Rt2 1.03. At a welding interface, an oxidation scale progresses in a direction away from an axis along side surfaces and changes the progressing direction at the end points so as to progress in a direction toward the axis along bottom surfaces. The progress of the oxidation scale is restrained. Since the noble metal tip is held by an engagement portion of a ground electrode, separation of the noble metal tip from the ground electrode 40 can be restrained.