PURPOSE: To perform graft-polymerization with an extremely easy and simple operation in high uniformity and efficiency by immersing various kinds of fibers in a polymerizable monomer liquid and irradiating with microwave.

CONSTITUTION: Various kinds of fibers such as natural fibers and synthetic fibers are immersed in graft-polymerization liquid such as an acidic aqueous solution of pH 1.8-3.0 containing a monomer such as 2-hydroxyethyl acrylate and hydroxypropyl methacrylate and irradiated with microwave to extremely easily perform the graft-polymerization in high efficiency. The process can perform the steps from the polymerization to the drying at a stroke and enables uniform polymerization and easy modification treatment.