An abnormal-pixel detecting device includes an image sensor and an image processor. The image sensor is configured to capture an image of a subject. The image processor is configured to calculate: a ratio between a first plurality of pixel values captured by the image sensor and a second plurality of pixel values whose reference position is shifted relative to the first plurality of pixel values in a main scanning direction to obtain a third plurality of pixel values; and detect an abnormal pixel in the third plurality of pixel values.