Provided are: a rubber composite in which cellulose nanofibers are well-dispersed in rubber and which has favorable characteristics (in particular, storage modulus, etc.) that make it possible to form a vulcanized rubber product having an excellent elastic modulus, excellent wear resistance, etc.; and a cellulose nanofiber-containing composition from which said rubber composite can be formed. Provided is a composition containing cellulose nanofibers and a nonionic surfactant, wherein the nonionic surfactant has, as a hydrophobic moiety, a group that has 6-30 carbon atoms and is an aliphatic or aromatic or a combination thereof.