An image forming apparatus (50) at least includes a paper output tray (80), an actuator (16) and a photo sensor (15). The actuator (16) is supported in such a manner as to be swingable above a paper loading surface of the paper output tray (80). The actuator (16) includes a fullness detection section (164) and a rear edge curling detection section (162). The fullness detection section (164) is disposed at a tip portion of the actuator (16), being configured so as to contact the uppermost paper on the paper output tray (80). The rear edge curling detection section (162) is formed so as to extend from the fullness detection section (164) toward a direction that gets further away from the paper loading surface of the paper output tray (80) toward a direction generally parallel to the paper loading surface of the paper output tray (80) and opposite to a paper conveying direction.