Provided are: a retardation-increasing agent that has excellent compatibility and, when incorporated into a resin film, is capable of not only adjusting the thickness-direction phase difference toward a positive direction but also imparting the film with high retardation; a cellulose-based resin composition comprising the retardation-increasing agent; and a film that is obtained from the cellulose-based resin composition and has excellent optical properties. The retardation-increasing agent comprises a compound represented by the following Formula (1) or (2): (wherein, R1 to R10 each independently represent a hydrogen atom or the like); or (wherein, R21 to R28 each independently represent a hydrogen atom or the like).