A lens-fitted photo film unit contains a photo film cassette, which includes a spool for winding the photo film in a roll form thereabout. A body tube contains the photo film and the spool in a rotatable manner, has a first end with a first end opening, and a second end with a second end opening. A first end cap closes the first end opening, and has an outer diameter d1. A second end cap closes the second end opening, has an outer diameter d2, and satisfies a condition of:

d2d1.

To contain the cassette, a cassette holder chamber in the lens-fitted photo film unit has a size corresponding to that of the cassette.