A paper feeder comprising an actuator and a sensor mounted on a main body, the actuator being capable of contacting an upper surface of a sheet placed on a bottom plate provided in a sheet tray as the bottom plate is raised, thereby allowing the sensor to detect that the sheet is present. The paper feeder further comprises a through bore formed in the bottom plate for receiving the actuator when a sheet is absent from the sheet tray, so that the sensor may detect that the sheet is absent, and a push-out guide provided on peripheries of the through bore for contacting the actuator in the through bore as the sheet tray is pulled out, thereby pushing the actuator upward out of the through bore.