A sealing agent stock solution 20 is prepared by preparing a mixed solution 14 by mixing a rubber latex, a resin emulsion and a surfactant, and a predetermined amount of water in a solution preparation vessel 12, followed by adding an anti-freeze agent 18 to the mixed solution 14. The sealing agent stock solution 20 is held (left standing still) at least over for 24 hr or more and preferably over for 48 hr or more without stirring in the solution preparation vessel 12, followed by filtering the sealing agent stock solution 20 with a filter member 26 made of a mesh filter having the mesh number from 50 to 400 mesh to remove latex aggregates 36 generated in the sealing agent stock solution 20, and thereby a sealing agent 10 that does not contain the latex aggregates 36 is produced.