The ratio of the particles of developing agent having a particle diameter of not more than A.times.0.5 (.mu.m) (wherein A is a 50% average particle diameter (.mu.m)) and the ratio of the particles of developing agent having a particle diameter of not less than A.times.1.5 (.mu.m) (wherein A is a 50% average particle diameter (.mu.m)) are both confined to not more than 5% by number in a number particle size distribution, the ratio of the developing agent having an adhesive strength of not more than 1.3.times.10.sup.-8(N) is confined to 10% by weight or less and the ratio of the developing agent having an adhesive strength of not less than 3.times.10.sup.-7(N) is confined to 5% by weight or less in a distribution of adhesive strength to the surface of the image carrier.