A waterproof casing contains a lens-fitted photographic film unit in a watertight manner so as to take underwater photograph. The waterproof casing is provided with an air flow opening. An air-permeable waterproof sheet is attached to the air flow opening form the inside so that air can pass through the sheet freely so as to regulate the internal pressure of the waterproof casing but water will not enter the waterproof casing. In a preferred embodiment, the waterproof casing consists of a transparent main body with a bottom opening and a bottom lid. The air flow opening is formed os as to be covered by a film winding knob of the waterproof casing. Structure to be fitted in the bottom opening is formed with another air flow opening to which the air-permeable waterproof sheet is attached. Outside this structure is disposed a waterproof sheet having an opening at a position facing the air flow opening of this structure. The waterproof sheet is covered by the bottom lid.