To effectively prevent toner from causing low-temperature offset in a color image forming process where several color toners are thermally fixed in an overlapped state.
In the thermal fixation process, four color toners, that is, yellow, cyan, magenta and black toners, are successively overlapped and transferred to a transfer material 6, and the black toner 13BK faces the fixing surface of a heat roller 8a. The isolation ratio of the additive agent of the black toner 13BK is set smaller than those of other color toners 13M, 13C and 13Y, and set to be the smallest. Then, the amount of the free additive agent of the black toner 13BK lying between the fixation surface and the black toner 13BK is reduced, and the low-temperature offset of the toner is prevented.
KADOTA TAKUYA
TAKANO HIDEHIRO
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