An image-forming device includes an image-forming unit, a sensor, a screening unit, an excluding unit, and a determining unit. The image-forming unit forms a plurality of marks on an object. The sensor detects a light reflected on the object, the reflected light including a plurality of waveforms. The screening unit screens the plurality of waveforms into a plurality of mark waveforms corresponding to the plurality of marks respectively and a plurality of noise waveforms other than the plurality of mark waveforms. The excluding unit excludes a mark waveform adjacent to the noise waveform from the plurality of mark waveforms. The determining unit determines a position of each mark based on the plurality of mark waveforms from which the mark waveform adjacent to the noise waveform is excluded.