A lens-fitted film unit has a film winding mechanism comprising a planetary gear set disposed between a winding knob and a rotary shaft engageable with a spool of a pre-loaded film cartridge to which a film strip at one end is attached. The planetary gear set comprises a sun gear as an integral part of the rotary shaft, a ring gear having a greater number of teeth than that of the sun gear, and at least one planet gear in mesh with the sun gear and the ring gear which revolves around the sun gear by means of rotation of the winding knob and rotates on its own axis so as to increase rotation of the winding knob and transmits the increased rotation to the rotary shaft, and hence the spool, so as thereby to decrease a burden on the user.