In a spark plug, an annular seal packing is arranged in a gap defined between an insulator and a fitting. Electrically insulating oil is filled in an oil-retaining section of the gap that is located on the distal side of the packing. The oil-retaining section of the gap is the continuous section having a width ranging from 0.05 mm to 0.3 mm and an axial length of 2 mm or more from the packing. The filled insulating oil is retained within the oil-retaining section of the gap by its surface tension while it is in a liquid state.