US Patent 7618760 Two-component developer

A two-component developer suppresses burying of an external additive into toner particles and a change in charging performance of a toner with time, and secures stable development performance. The two-component developer includes a toner and a carrier and is adjusted so that a rate of change ΔA % in BET specific surface area A (m2/g) of the toner (=((A120−A1)/A1)×100) and a rate of change ΔC % in charge quantity C (μC/g) of the carrier (=((C120−C1)/C1)×100) when a mixture of the toner and the carrier is stirred and mixed are both within 10 %. A1and A120 indicate BET specific surface areas of the toner when the mixture of the toner and the carrier is stirred and mixed for 1 minute and 120 minutes, and C1 and C120 indicate charge quantities of the carrier when the mixture is stirred and mixed for 1 minute and 120 minutes.