To obtain a watertight function that is stable by means of a single watertight cover regardless of the size of an opening.

The watertight tank includes a tank body 1 for placing a generator B thereon; a plurality of lids 2 pivotally supported on its open edges so as to be freely inclined in the inward/outward direction of the tank body 1; and waterproof covers 3 disposed so as to project from the side edges 20 of the corresponding lids 2. Each waterproof cover 3 integrally has a non-deformable portion 3A and a watertight elastically deformable portion B. Each non-deformable portion 3A is supported by the corresponding lid 3 so as to projectingly slide from the side edge of the lid 2. The elastically deformable portion 3B is supported by the non-deformable portion 3A so as to project in the sliding direction from the edge of the non-deformable portion 3A, which is located in the sliding direction. The amount of projection of the elastically deformable portion 3B is set so that the elastic deformation by its weight is restricted when the closed lid 2 is inclined to the maximum to be elastically deformed by overlapping the side edge 20' of the lid 2' when the closed lid 2 is inclined most from when it is inclined to the minimum.

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