To produce a water-soluble hot melt adhesive excellent in heat stability in a molten state.

This water-soluble hot melt adhesive contains a vinylpyrrolidone- vinyl acetate-based copolymer. A molar ratio of the vinylpyrrolidone to the vinyl acetate in the vinylpyrrolidone-vinyl acetate-based copolymer is (the former/the latter) of about (85/15)-(10/90). The water-soluble hot melt adhesive may contain (a) about 50-300 pts.wt. of a water-soluble or a water-dispersible tackifier (a rosin derivative, etc.), (b) about 10-120 pts.wt. of wax (wax oxide, etc.) and (c) about 40-300 pts.wt. of a water-soluble or a water-dispersible plasticizer (glycerol, etc.) per 100 pts.wt. of the vinylpyrrolidone-vinyl acetate-based copolymer. The adhesive is useful as a lap paste for cigarette filter paper.