Abstract: A diesel engine preheating apparatus connects a plurality of glow plugs arranged in corresponding cylinder chambers of a multicylinder diesel engine in series or parallel with or in series and parallel with a power source. The glow plugs are selected from body-grounded type unipolar and bipolar glow plugs. The bipolar glow plug selected from the diesel engine glow plugs includes first and second external connecting terminals which are respectively connected to thin-walled portions of first and second terminal connecting plates located on substantially the same plane. The thin-walled portions interpose an insulating member therebetween, are located at opposite surface sides, and are provided with portions which do not overlap each other when viewed from an upper surface.