PURPOSE: To improve the moisture-proof performance by kneading a resin composition consisting of a specified amount of an inorganic filler formulated with a thermoplastic resin using a kneader, then further kneading and deaerating the mixture using an extruder, and coating the extruded product.

CONSTITUTION: A resin composition is prepared by blending 100 pts.wt. of a thermoplastic resin component such as polyethylene with 20 to 80 pts.wt. of an inorganic filler such as talc, and the blend is kneaded using a kneader. Next, the blend is kneaded and deaerated using an extruder. A thermoplastic resin composition for the formation of a moisture-proof resin coating film formed as described is coated. The resin composition used has a density of at least, about 1.0g/cm3. If the formulation dosage of the inorganic filler is 20 pts.wt. max, the adequate efficacy of the filler cannot be achieved. On the other hand, if the dosage is 80 pts.wt. min, the binding action of the thermoplastic resin is inadequate.