A document feeder includes a frame to be disposed on a platen of a scanner; a sheet feeding stacker, a discharge stacker, and a transporting belt mounted on the frame; a lateral positioning member mounted on the frame for restricting the transporting belt laterally; and a vertical positioning member mounted on the frame for restricting the transportation belt vertically. The lateral positioning member is composed of a projection for abutting against a flange portion provided at a peripheral side of the platen to laterally restrict a position of the transporting belt. The vertical positioning member is composed of a projection for abutting against a surface of the platen to vertically restrict the position of the transporting belt.