The present invention relates to cellulose nanofibers that have good dispersibility in water and can be contained in a water-soluble polymer at high concentration, and to a method for obtaining the cellulose nanofibers. The present invention further relates to a fiber reinforced composite material using the cellulose nanofibers. Cellulose nanofibers having an average degree of polymerization of 100 or more and 800 or less and an aspect ratio of 150 or more and 2000 or less are produced by subjecting unmodified cellulose to an enzyme and/or acid treatment, and to a mechanical shearing treatment. The cellulose nanofibers have good dispersibility and can be dispersed in a water-soluble polymer at high concentration, and therefore a fiber reinforced composite material having high strength can be obtained.