To reduce the generation of free formaldehyde from a processed fiber product without hardening the finished feeling when processing a cellulosic fiber by using methyloldihydroxyethyleneurea-based compound.

This fiber-treating agent contains the methyloldihydroxyethyleneurea-based compound methylolated within the range of 1.4-2 mol per 1 mol of an ethyleneurea ring, and. 5-40 pts.wt. of a polyol having ≤200 molecular weight, and ≥5 pts.wt. polyethylene glycol having ≥1,000 average molecular weight per 100 pts.wt. of the methyloldihydroxyethyleneurea-based compound. The method for processing the cellulosic fiber comprises applying a treating bath containing the fiber- treating agent and an acidic catalyst to the cellulosic fiber within the pH range of 2.5-5 at 25°C.