Resistor compositions for producing a resistor for use in resistor-incorporated spark plugs and comprising a glass, an inorganic aggregate, and carbon black and/or a carbonaceous substance as a regulating agent for resistance value, with or without a stabilizing agent for resistance under load of the resistor, are now modified by addition of a reducing metallic powder such as powder of Al, Mg, Zn, Sn, Ti, Zr, Fe, Ga and Ag or alloys thereof, to improve the service life characteristics under load of the resistor. Such compositions are further modified by additional presence of a ceramic aggregate of a particle size of 100 to 800 .mu.m, to improve the electric wave noise-preventing property.