There is provided with an apparatus. A measurement unit emits light to an image carrier conveying a toner image and measures an incident position of the light in a direction perpendicular to a surface of the image carrier by observing the emitted light. The measurement unit has an irradiation unit configured to emit the first light and the second light and an observation unit configured to observe the first light and the second light. The irradiation unit emits the first light and the second light at positions separate from each other in a direction perpendicular to a conveyance direction of the toner image, with irradiation directions of the first light and the second light being substantially orthogonal to the direction perpendicular to the conveyance direction of the toner image.