Disclosed is a developing agent, wherein temperature T2, at which the developing agent exhibits a viscosity of 0.5×105 (Pa·s) after receipt of a thermal history that, after once being heated at a prescribed temperature, the developing agent is cooled to room temperature, falls within a range of 100 to 140° C., temperature T1, at which the developing agent exhibits a viscosity of 0.5×105 (Pa·s) before receipt of the thermal history, falls within a range of 100 to 150° C., and the difference T2−T1 is 0 to 10° C.