To provide a lens-fitted photo film unit with a built-in exclusive battery protected from repacking.

A unit main body 3 of the lens-fitted photo film unit 2 comprises a main body base 19 to which an exposure device 16 and an electronic flash device 17 have been attached, and a front cover 20 and a rear cover 21 which cover the front and rear of the base 19. In the main body base 19, a cartridge chamber 22, a film roll chamber 23 and an aperture 24 are formed. The rear side faces of the chambers 22, 23 and the aperture 24 are closed up with the rear cover 21 to form a light shielding space 45. A battery 41 is built into a part under the aperture 24. In the main body base 19, protrusions 19a, 19b are formed in one body. The protrusion 19a is located near the positive pole terminal 42 of the battery 41 and has a hollow structure. The hollow 50 of the protrusion 19a communicates with the light shielding space 45.