Provided is a modified blowing agent, wherein the surface of a blowing agent powder is coated with a metallic siloxylated compound. When the modified blowing agent is included in a polymer resin, dispersibility of the modified blowing agent is excellent in the polymer resin and fluidity of the polymer resin is improved in an extrusion molding machine and an injection molding machine, and thus productivity and processability can be improved. In addition, the foam that is formed using the polymer resin composition including the modified blowing agent has excellent tear strength, durability, and masking sound properties due to fine and uniform size of foam cells and increased number of foam cells per unit surface area.