As a process for producing a water-resistant organic thin film, a process of applying a water-resistance-imparting treatment solution on the surface of an organic thin film laminated on one surface of a base material is known. However, the organic thin film before the water-resistance-imparting treatment is very brittle, and is often delaminated or cracked upon the contact with a support means such as a roll. In addition, the organic thin film might be cracked merely by bending the base material. Further, the organic thin film might be delaminated by showering the water-resistance-imparting treatment solution thereon. Thus, it has been difficult to carry out the water-resistance-imparting treatment continuously without causing damage to the organic thin film. By the employing the following techniques (a) to (c), it becomes possible to carry out the water-resistance-imparting treatment continuously without causing damage to the organic thin film: (a) the support means is so arranged as not to contact with the organic thin film; (b) the base material having the organic thin film laminated thereon is not bent; (c) the base material having the organic thin film laminated thereon is allowed to pass through the water-resistance-imparting treatment solution in a reaction vessel gently.