A lens-fitted photo film unit is pre-loaded with photo film (31). An exposure is taken through an exposure opening (43) upon depression of a shutter button (14) to a full depressed position. A halfway position of the shutter button is less deep than the full depressed position. A power source switch (44) is switched on upon movement of the shutter button to the halfway position. A photometric circuit (50, 60) receives light of an object, and for generating a changeover signal when brightness of the object is a predetermined brightness or more. The photometric circuit is rendered effective when the power switch is switched on. A changeover plate (46a) has a stop-down opening (46b) which is formed therein, and has a size smaller than the exposure opening. The changeover plate is movable between an inserted position and a retracted position. The stop-down opening of the inserted position is superposed on the exposure opening to intercept a periphery of the exposure opening. The stop-down opening of the retracted position is away from the exposure opening. The changeover plate is set in the inserted position upon occurrence of the changeover signal and in response to movement of the shutter button to the full depressed position from the halfway position. The changeover plate is set in the retracted position in absence of the changeover signal.