To synthesize a high-strength and heat-resistant novel cellulose-based bioplastic having a long side chain with low energy consumption, we developed a novel two-step heterogeneous process. The bioplastic is a cellulose ester synthesized by bonding a short side chain (acetic acid) and a long one (3-pentadecylphenoxy acetic acid, a derivative of cardanol, extracted from cashew nut shells). In conventional homogeneous processes, cellulose esters are recovered by precipitation with large quantities of poor solvents, which requires much energy consumption for their distillation. In the novel process, first, limited amounts of these chains are bonded in a heterogeneous system to achieve efficient product recovery by filtration without precipitation. Second, the short-chain acid is additionally bonded to attain good thermoplasticity of the final product, the cellulose resin, which is recovered by distilling the reaction solvent and the remaining short-chain component. The solvent usage was reduced by approximately 90% compared with a homogeneous process. The thermoplasticity of the resulting resin was comparable to that of a homogeneous one. Furthermore, the mechanical and thermal characteristics of the resin were greatly improved by adding a specific linear polyester, poly(butylene succinate adipate), and a glass fiber, achieving high target levels for durable products.