A substantially rectangular insertion opening is formed on the upper surface of the housing of the radiation image reading device. An L-shaped projecting portion which runs from the back side periphery to the right side periphery of the insertion opening is integrally formed with the housing and the projecting portion protrudes above the upper surface of the housing, and the top surface of the projecting portion is higher than the upper surface of the housing. When the cassette is inserted into the insertion opening, if the cassette is allowed to come in contact with the side surface of the projecting portion, positioning of the cassette at the insertion opening is simpler. As a result, quicker and easier insertion of the cassette into the insertion opening becomes possible.