PURPOSE: To prevent rain water from accumulating at the roof center of a waterproof cap by putting the cap with a space to store an air bag at the rear center of a roof of it on a waterproof sheet which covers a building unit, etc., under work, and inflating the air bag in the space to increase the roof height of the cap.

CONSTITUTION: A waterproof cap 3 with a space to store an air bag 4 is put on a waterproof sheet 2 which covers a building unit 1, the side of the cap 3 is pulled down to reduce looseness, and the cap is fixed. Next the air bag 4 is inflated so that the roof height of the cap is reduced gradually from the center part to the periphery of it to prevent rain water from accumulating at the roof center of it. When the cap is removed after use, the air bag 4 is removed and folded flexibly. Thus a waterproof cap with good workability can be obtained.