To provide a cover for a water treating vessel on which a flat planar solar cell panel can be easily mounted even if the top surface of a cover body curves convexedly.

The cover 1 for the water treating vessel 10 has a cover body 2 which covers an opening 11 in the upper part of the water treating vessel 10 the top surface 21 of which curves convexedly and the solar cell panel 3 which is installed on the top surface 21 of the cover body 2. The cover is provided with a holding member 5 for holding the solar cell panel 3 between itself and the cover body 2 by a pressing force of an elastic member 4 arranged at the end of the panel.