Provided herein is a cellulose nanofiber-containing composition producing method for producing a cellulose nanofiber-containing composition that can be easily combined with compounds having a reactive double-bond group, and that contains only a small amount of uncured material that acts as a plasticizer in a molded product, using a simple producing method that does not require any process involving solvent displacement or solvent removal. A high-strength molded body prepared by using the cellulose nanofiber-containing composition is also provided. In refining cellulose in a mixture containing a compound having a reactive double-bond group and a hydroxyl group of 10 KOHmg/g or less and a defibrating resin as essential components, the cellulose has a moisture content of 4 to 25 parts by mass with respect to 100 parts by mass of the amount of the cellulose converted on the assumption that the percentage moisture of the cellulose is 0%.