An apparatus to form plural images on an image conveyor includes plural photoconductor bodies, a light exposure device, a development device, a transfer device, and a pattern detection device, wherein the light exposure device includes an optical scanning part, a first light detection part having a first light receiving face, a second light detection part having a second light receiving face being non-parallel to the first light receiving face, a third light detection part having a third light receiving face being non-parallel to the first light receiving face, the optical scanning part, the first light detection part, and the second light detection part being provided inside a housing of the light exposure device, the third light detection part being provided outside the housing, and a light exposure timing control device.