To obtain a new polyolefin-based polymer utilizable for an adhesive, a hot-melt composition, a water-dispersible hot-melt composition or a moistureproof paper and excellent in fluidity and temperature characteristics and the adhesive, hot-melt composition and the recyclable moistureproof paper using the polyolefin-based polymer.

This polyolefin-based polymer is obtained by the following (X treatment) and/or (Y treatment). (X treatment): a mixture comprising an amorphous polyolefin and a random polypropylene is treated with a free radical generator capable of generating radicals within a specific range and (Y treatment) a mixture comprising the amorphous polyolefin and random polypropylene is treated with the free radical generator capable of generating radicals within a specific range and 0.0005-1 mol of an acid anhydride. The hot-melt adhesive and moistureproof paper comprise the polyolefin-based polymer.