An object of the present invention is to provide a β-1,3-glucan derivative which is a polymer having β-1,3-glucan as a main chain, and has thermoplasticity and excellent moldability, and a preparation method thereof. That is, the present invention provides a β-1,3-glucan derivative having a structure represented by General Formula (1) as a main chain.(In Formula (1), each of a plurality of R1s independently represents a hydrogen atom or —COR2, n represents an integer of 1 or greater, and R2 represents an aliphatic hydrocarbon group or an aromatic hydrocarbon group. In Formula (1), a plurality of the R1s may be the same as or different from each other, and at least a part of a plurality of the R1s is —COR2.)