Impact charging experiments were systematically conducted using four kinds of polymers for impacting particles and three metal targets with different work functions in all of their combinations. The equilibrium charges were independent of the work function of the metal target and were almost constant for each polymer. Discussion of the results of this experiment suggested that the charge relaxation in the particle separating process determimes the impact charge if this impact charging is generated under atmospheric conditions. One important reason for studying this charge relaxation process is to understand the particle charging mechanism. Furthermor, these discussions and experimental results suggested that the characteristic which decides the amount of charge generation, in charge generation in the dry powder processes etc., is the dielectric constant of the polymer material of which the particle consists.