An image forming device equipped with a light scanning device having a polygon mirror that deflects laser light by rotating, a focusing element for focusing deflected laser light onto a photosensitive body, and a reflection mirror reflecting and guiding to the photosensitive body the laser light which passes through the focusing element, given that a distance L between an end surface of the focusing element and the reflection mirror is 8 mm.ltoreq.L.ltoreq.25 mm, and an angle .THETA. formed by incident light and reflected light of the reflection mirror is 25.degree..ltoreq..THETA..ltoreq.45.degree., the focusing element, the reflection mirror, and the photosensitive body are disposed such that a relationship between L and .THETA. satisfies 8 mm .ltoreq.Ltan.THETA..ltoreq.12 mm is provided.