To provide an adjusting method with which the resistance value of a resistance-heating element, formed into a planar or a layer form on a radiating substrates, such as a ceramic board and a glass board is readily adjusted, while maintaining a stable exothermic state.

The resistance adjustment of the resistance-heating element 2 formed in layers on the radiating substrate 1 of the ceramic board or the glass board is adjusted by preparing excised sections 5 on the face area of the resistance-heating element, such as a slot of which the resistance-heating element is partially excised. The excised section is formed into a planar form, which has aspect ratio of the surface shape of the resistance-heating element. An overcoat 6 of an insulator is formed in the surface of the resistance-heating element, and the excised section is prepared in the resistance-heating element from the surface of the overcoat.