To provide an image forming device and a developer amount informing system capable of preventing the erroneously informed value of developer amount in a period when the detected result of the developer amount is not stable and always informing the developer amount stably.

In this image forming device to/from which a cartridge is attached/detached and which is provided with a measuring means 43 measuring output from a developer amount detection means 50, a calculation means 113 calculating the developer amount based on the measured value by the measuring means, and a control means 113 controlling information on the calculated result on the developer amount by the calculation means, the control means is constituted to control to inform of a specified state set in advance regardless of the calculated result by the calculation means 113 during the specified period set in advance.