A tandem-type image forming device is provided which includes a plurality of photosensitive drums, a paper edge detecting section to detect a front edge and a rear edge of a sheet of paper, an exposure starting time calculating unit for each of a plurality of photosensitive drums, an exposure inhibiting time calculating unit to inhibit printing, when paper having a length being smaller than a predetermined length of the paper is inserted, on a portion being shorter than preset paper, and a control section to control an entire image forming device. By configuring as above, the tandem-type image forming device can be realized by using only one paper edge detecting sensor and only one detecting circuit. Even when paper having a length being smaller than a preset length of the paper is inserted, no transfer of an image to a place outside of the paper, that is, to a carrying belt occurs.