To provide a water-based resin composition for a sealer, free from chlorine and organic solvent, giving extremely little load on the environment and suitable for the sealing of a zinc-plated steel plate.

The objective water-based resin composition for a sealer suitable for steel plate and giving minimized load on the environment is produced by adding an inorganic filler, an organic thickener or their mixture to a water- based resin consisting of an aqueous emulsion polyolefin resin composed of a polyolefin free from chlorine as a constituent element and having a viscosity of ≥1,000 mPa.s in a state dispersed in water at a solid resin concentration of 20-40 wt.% thereby adjusting the viscosity of the resin composition to 5,000-100,000 mPa.s. A composition having extremely excellent corrosion resistant and corrosion proofing performance to a zinc-plated steel plate can be produced by adding a water-soluble magnesium compound to the resin composition.