To provide a water-proof case which has high watertightness at a low cost.

In a film unit with a lens in which a photographic film is preloaded, a unit main body comprises two case members; and one case member is a main body case 20 made of transparent laser-light transmission plastic and the other case member is a case back lid 21 made of opaque laser-light absorbing plastic. The main body case 20 and case back lid 21 are provided with joint parts 20a and 21a entirely at their outer circumferences respectively and the joint parts 20a and 21a are put one over the other; and the joint part 21a of the laser-light absorbing plastic is irradiated with laser light 40 from the side of the joint part 20a of the laser-light transmission plastic and the laser light is transmitted through the joint part 20a, and absorbed and changed into heat at the joint part 21a to fuse the surface of the joint part 21a with fusion heat, so that the joint parts 20a and joint part 20a are joined together.