To efficiently produce nanofiber sufficiently micronized and suppressed in reaggregation, from a cellulosic fiber material.

A polybasic acid half esterified cellulose is prepared by half esterifying a polybasic acid anhydride (especially a dibasic acid anhydride) at 140C to introduce a carboxy group into a part of the hydroxy groups of cellulose. The polybasic acid half esterified cellulose is microfiberized to produce nanofiber. Due to the presence of carboxy groups having a negative electric charge and introduced on the microfibril surface, repulsion between microfibrils is induced to stably disperse the microfibrils in the dispersion. Introduction of carboxy groups is easily performed by a simple reaction.