An image forming apparatus includes a sensor member having first and second ends opposite to each other in a longitudinal direction of the sensor member and supported by support structures consisting of the first and second ends, and first and second support portions supporting the first and second ends; a first support structure including a first protruding portion fitted on a first recess portion supporting the first protruding portion; and a second support structure including a second protruding portion fitted on a second recess portion supporting the second protruding portion. The second protruding portion has first and second cylinder portions extending in a longitudinal direction of the second protruding portion. The second recess portion has an insertion portion allowing the second protruding portion to be inserted, a pass portion allowing the first cylinder portion to pass therethrough, and a positioning portion fitted on the second cylinder portion.