The invention defines an adhesive force characteristic of a toner, which makes it possible to control behaviors of toner particles by applying an electric field, and defines, when an amount of charges before transfer per weight of the toner particles is Q (muC/g), an average adhesive force of the toner particles to the medium is F (N), a volume average particle diameter of the toner particles is d (mum), and a specific gravity of the toner particles is rho (g/cm<3>), an A value represented by A=(KxQ+F0/Q)x 6 /rhopid<3>) (K: an inclination at the time when F is approximated to a linear function of Q<2>, F0: a y intercept at the time when F is approximated to a linear function of Q<2>) in a range of 1x10<7><=A<=2.5X10<7 >(N/C). Consequently, a developing agent has a highly efficient and stable transfer characteristic and it is possible to form a high-quality image.