A highly adhesive pressure sensitive adhesive composition includes at least (A) a hydroxyl-group-containing acrylic polymer, (B) an amine compound containing plural hydroxyl groups, and (C) a polyisocyanate compound and has a gel fraction of equal to or more than 10% by weight and less than 70% by weight after drying or curing. A highly adhesive pressure sensitive adhesive sheet has a highly adhesive pressure sensitive adhesive layer including the highly adhesive pressure sensitive adhesive composition formed on one or both sides of a base material. The sheet preferably has a 180.degree. peel force with respect to a stainless steel sheet of 10 N/20-mm or more as determined at a pulling rate of 300 mm/minute, 23.degree. C. and 50% relative humidity.