In a spark plug for an internal combustion engine oil is applied onto at least either the surface of an intermediate body portion of an insulator or the surface of an intermediate hole portion of a metallic shell, thereby forming an oil film. Since the oil film has a dielectric constant falling between those of the insulator and an ambient air layer, a great change in dielectric constant between the insulator and the ambient air layer is eased. Also, since the oil film is of liquid, the oil film smoothes a dent or protrusion or a fine defect such as crack present on the surface of the intermediate body portion of the insulator. Thus, electric field concentration is suppressed, thereby preventing dielectric breakdown caused by such a dent, protrusion or fine defect.