Modified cellulose nanofibers and rubber composition containing same

The objective of the present invention is to provide a rubber composition which has sufficient reinforcing properties and fatigue resistance even in cases where a large strain is applied thereto. The present invention provides: substituted carboxyl group-containing modified cellulose nanofibers wherein at least some of carboxyl group-containing modified cellulose nanofibers have at least one of a substituent represented by formula (a) -CONH-R 1 and a substituent represented by formula (b) -COO-R 1 (in formulae (a) and (b), each R 1 independently represents a hydrocarbon group having at least one unsaturated bond and 3-30 carbon atoms); and a rubber composition which contains the substituted carboxyl group-containing modified cellulose nanofibers.