The development of biomass plastics, which are produced from plant biomass as a renewable resource, rather than petroleum, is expected. So far, biomass plastics have been made from materials such as polylactic acid, microbial polyesters, and bio-polyethylene derived from glucose, and ester derivatives of cellulose, a component of wood [1]. Although there exist high-molecular-weight polysaccharides with various structures other than cellulose in nature, many of them are very difficult to extract, and there are many challenges in using them as raw materials for plastics, such as small molecular weight, complex combination of multiple constituent polysaccharides, and difficulty in obtaining a homogeneous structure. Therefore, the development of a technology to synthesize high-molecular-weight polysaccharides with controlled molecular weight, such as glycosidic bond position and completely linear molecular chain structure, in vitro enzymatic polymerization is desired.