PURPOSE: To obtain an image forming device which is capable of suppressing the increase of the number of development persons and putting a main body composed of an option unit and a separated image forming means on sale without waiting for the development of the option unit to be prepared.

CONSTITUTION: A CPU performing a control for an image forming part 1 has a program area 41 for option control where the program for control of an option unit is reloadably written. The CPU mounted on an option cassette which is attachable and detachable to the image forming part 1 is connected with the CPU of the image forming part so that communication may be possible and has a program area 44 for option control where a program for control which is required for the control of the option cassette by the CPU of the image forming part is stored. The CPU of the option cassette transfers the program for control which is stored in a program area 44 for option control to the CPU of the image forming part 1.