PURPOSE: To prevent interference between an actuator and paper when mounting/dismounting a paper feed tray against the copier body by forming a slope for lifting/lowering the actuator in the paper feed tray at the position to be engaged with the actuator for functioning a paper detecting member.

CONSTITUTION: The paper size detector 20 is comprised of an actuator 3 having one end contactable with the upper face of paper 2 contained in paper feed tray 1 and a detection member or microswitch 4 coupled to non-contact end of actuator 3. The actuator 3 is rollable around a pivot 5 where a roller 6 is provided at the end section contactable with the upper face of paper 2 while sliding section 7 is provided at the portion to be slided with the paper feed tray 1. In such detector, slopes 9a, 9b for lifting/lowering the actuator 3 are formed at the upper edge of rear frame 8 in paper feed tray 1. Consequently, when mounting/dismounting the paper feed tray 1 against the copier body, the actuator 3 is retracted upward.