A document reading unit comprising: an original document table on a surface of which an original document is placed; a moving irradiation unit having multiple block light sources aligned in a main scanning direction of a surface of the original document and to move in a sub scanning direction that is a direction along the surface of the original document table and perpendicular to the main scanning direction while irradiating light emitted by the multiple block light sources to the original document placed on the original document table and obtaining reflected light reflected on the original document; an image sensor to receive reflected light from the original document irradiated by the moving irradiation unit and read an image formed on the original document; and a controller configured to perform a length specifying operation to specify a length of the original document in the main scanning direction, wherein the controller is configured to start the length specifying operation in a state in which the moving irradiation unit is located at a position facing the original document placed on the original document table, sequentially turn on the multiple block light sources one by one, obtain an amount of reflected light received by the image sensor under respective lighting conditions of the multiple block light sources, and determine the length in the main scanning direction of the original document based on the obtained amount of reflected light, wherein the controller is configured to cause the image sensor to receive the reflected light reflected on a region (S0, S1, S2, and S3) having a width in the sub scanning direction of the original document placed on the original document table under the multiple lighting conditions during the length specifying operation while causing the moving irradiation unit to move in the sub scanning direction, and characterized in that, in the length specifying operation, the controller is configured to turn on a first block light source of the multiple block light sources to obtain the amount of reflected light by the image sensor in a unit moving time period in which a speed of movement of the moving irradiation unit is increasing to a given speed after the start of movement of the moving irradiation unit. an original document table on a surface of which an original document is placed; a moving irradiation unit having multiple block light sources aligned in a main scanning direction of a surface of the original document and to move in a sub scanning direction that is a direction along the surface of the original document table and perpendicular to the main scanning direction while irradiating light emitted by the multiple block light sources to the original document placed on the original document table and obtaining reflected light reflected on the original document; an image sensor to receive reflected light from the original document irradiated by the moving irradiation unit and read an image formed on the original document; and a controller configured to perform a length specifying operation to specify a length of the original document in the main scanning direction, wherein the controller is configured to start the length specifying operation in a state in which the moving irradiation unit is located at a position facing the original document placed on the original document table, sequentially turn on the multiple block light sources one by one, obtain an amount of reflected light received by the image sensor under respective lighting conditions of the multiple block light sources, and determine the length in the main scanning direction of the original document based on the obtained amount of reflected light, wherein the controller is configured to cause the image sensor to receive the reflected light reflected on a region (S0, S1, S2, and S3) having a width in the sub scanning direction of the original document placed on the original document table under the multiple lighting conditions during the length specifying operation while causing the moving irradiation unit to move in the sub scanning direction, and characterized in that, in the length specifying operation, the controller is configured to turn on a first block light source of the multiple block light sources to obtain the amount of reflected light by the image sensor in a unit moving time period in which a speed of movement of the moving irradiation unit is increasing to a given speed after the start of movement of the moving irradiation unit.