To facilitate the coupling work of the cap member of a clean water storage tank by providing the clean water storage tank with a rotation detaining mans capable of coupling the cap member disposed separably on the upper side of the clean water storage tank by rotating the cap member at a prescribed angle.

The upper side of the clean water storage tank is provided with the cap member 70 and the inner peripheral surfaces of the clean water storage tank and the cap member 70 are provided with the rotational detaining means 100 capable of coupling or separating the cap member by rotating the cap member at the prescribed angle. The rotational detaining means 100 consist of at least ≥2 detaining grooves 110 on the outer peripheral surface of the connector of the clean water storage tank and detaining projections 120 inserted into these detaining grooves 110 on one side of the inner peripheral surface of the cap member 70. The detaining grooves 110 are formed of slender first projecting parts 111 and first fixing projections 112 for preventing the detachment of the detaining projections 120 after the insertion at one end of the first projecting parts 111. On the other hand, the detaining projections 120 are formed of slender second projecting parts 121 and second fixing projections 122 for preventing the detachment of the second detaining projections 121 from the detaining grooves 110.