PURPOSE: To obtain a transparentizing agent for paper for an envelop with a window, containing no chlorine-based solvent, not hindering recycle of paper.

CONSTITUTION: This agent for transparentizing paper for an envelop with a window is an aqueous emulsion type obtained by mixing 100 pts.wt. of a carboxyl group-containing thermoplastic resin with 1-10 pts.wt. of a surfactant, 20-200 pts.wt. of water, 0-70 pts.wt. of a solvent and 0-1.0 pt.wt. of a silicone- based anti-foaming agent or mold releasing agent. Another aqueous emulsion type agent is alternatively obtained by mixing 90-100 pts.wt. of a carboxyl group-containing thermoplastic resin with 10-90 pts.wt. of a thermoplastic resin containing no carboxyl group to give a resin and blending 100 pts.wt. of the resin with 1-20 pts.wt. of a surfactant, 20-200 pts.wt. of water, 0-70 pts.wt. of a solvent, 0-1.0 pt.wt. of a silicone-based anti-foaming agent or mold releasing agent and 0-25 pts.wt. of a wax.