This invention aims to develop a chitin oligomer composition and/or a chitosan oligomer composition wherein the content of an oligomer from a trimer to a decamer is 60% or more, and to develop a process for preparing the above-mentioned oligomer composition. The object of the invention can be achieved by mixing hydrofluoric acid or hydrohalogenated acid other than hydrofluoric acid and hydrofluoric acid with a chitin-based material and/or a chitosan-based material.