The present invention relates to a novel inulin synthase having a function and a substrate specificity of acting on sucrose to produce inulin, but not acting on kestose, maltose, lactose, trehalose and cellobiose; and a process for producing inulin comprising the step of allowing the synthase, a culture fluid or cultured cells of a microorganism producing the synthase, or a treated product thereof to contact with sucrose to produce inulin.