In a mercury-free arc tube, electrodes are configured by electrode rods heat-treated at 1,200 to 2,200.degree. C. in a vacuum atmosphere, and which have a thickness of 0.3 mm or more. The size (radius R) of a residual compression strain layer formed around the electrode rod is set to be 1/4 or less of the width D of the pinch seal portion. A glass layer outside the residual compression strain layer is thickened, thereby enhancing the strength of the pinch seal portion.