A glow plug is provided in a combustion chamber of a diesel engine. The tubular composition to make the return passage is composed of a tubular skeleton to guide the rollers, and a porous compact impregnated with lubricant. The input of a second amplifier amplifying one of the differential signals is connected to one signal terminal on the balanced side of the balanced-unbalanced converter. Juvenile wood fibers are the preferred source of fibers having low coarseness.