To provide an X-ray film developing unit capable of immediately and easily developing the X-ray film photographed by a portable X-ray photographing device and to provide a method for using the developing unit.

A bag 1 is constituted of sheets 11 and 12 through which light is not transmitted, but X-ray is transmitted. In the bag 1, a film chamber 1A is provided on the base part 13 side, and a developer chamber 1B is provided on an opening 14 side, and both chambers 1A and 1B are partitioned by a light shielding part 15. A light shielding part 16 is provided near the opening 14. The X-ray film F and an intensifying paper 2 are inserted into the film chamber 1A. Light is shielded by the light shielding part 15, or 16, then, the X-ray photographing is executed, and the intensifying paper 2 is pulled out with a black box, and light is shielded by the light shielding part 15, and developer is injected into the chamber 1B outside the black box, then, the light is shielded by the light shielding part 16, and the light shielding part 15 is opened so that the developer may flow into the chamber 1A, then, the X-ray film F is developed.