To provide an image forming device capable of preventing the transfer drop-out image from being produced.

This device is provided with a flapping detection means for detecting flapping 52b of a carrying belt, consisting of a flapping detection roller 71 arranged on the carrying belt surface leaving a specific gap and used as a surface moving member at least the surface of which is movable and of a movement detecting means for detecting whether the surface moving member is moved or not. Thus, in accordance with the detection result of the presence or absence of the flapping, a specified processing for preventing the above transfer paper from being carried over a section that the flapping is present can be executed, therefore the transfer drop-out image can be prevented from being produced by restraining the above transfer paper being carried to the section that the flapping is present.