PURPOSE: To allow safely using a mouth holder of waterproof umbrella sacks anywhere without anxiety and the need of a power source.

CONSTITUTION: A waterproof umbrella sack 1 consists of a front sheet and a back sheet which are formed into a bag-like shape, and the upper end of the back sheet is extended beyond the upper edge of the front sheet to form a tongue flap at top of the sack 1. A mouth holder has a storage compartment 2 where the two or more waterproof sacks 1 are stored, an opener member 3 to be inserted into the waterproof sack l in the front row along the tongue flap to open the mouth of the waterproof sack 1, and a driving member 4 that drives the opener member 3. The driving member 4 is situated in a sack draw- out path 5, and consists of a driving lever 4A which is moved by an umbrella K that is pulled out and a power transmitting member 4B which transmits the motion of the driving lever 4A to opener member 3 so that the opener member 3 is driven by the driving lever 4A to open the mouth of the waterproof sack 1.