To try to modify the surface of fine powder by grinding by a grinder and supplying liquid surface treatment agent immediately after classification by a classification means.

A classification means 20 which is installed next to a grinder and classifies fine powder ground by the grinder using gas such as air, nitrogen gas, an carbon dioxide as a medium and a spray means 30 for supplying a surface treatment agent to the fine powder classified by the classification means 20 are provided. The spray means is made to face the classification means 20, the surface of the fine powder which was ground by the grinder and classified by the classification means 20 is modified.