Hydroxypropyl methylcellulose (HPMC), a cellulose derivative, is highly water soluble, viscoelastic, and thermoplastic. However, the thermoplasticity of HPMC has not yet been studied in detail, which may be because HPMC deteriorates at a high temperature. In this study, the thermal extrusion of cellulose using HPMC is investigated with the aim of molding cellulosic biomass without petroleum-based resins or starch. The effects of the extrusion load, plasticizer, and mixing ratio of cellulose to HPMC on extrusion are evaluated using a capillary rheometer at a heating rate of 2 °C/min. Glycerol is found to be an effective plasticizer for HPMC. Cellulose is successfully extruded at approximately 200–230 °C. The extrusion temperature can be lowered by increasing the extrusion pressure and the quantities of glycerol and HPMC, to prevent the thermal decomposition of HPMC and hemicellulose in the cellulosic material. Even a mixture with a cellulose: HPMC ratio of 8:2 can be thermally extruded. Thus, HPMC is found to be a thermoplastic cellulose derivative that is controllable and can be used to extrude cellulosic materials to develop an all-cellulosic composite material.