Stephania japonica Miers (syn. Stephanin hernandifolia Walp.) is a climbing plant native to the southern regions of Japan. In traditional medicine, it is called "Qianjin藤" (Qianjin vine) and has been used to treat consumptive diseases, malaria, and miasma, while its stems are utilized for basket weaving in folk practice. Previously, Mr. Kubomi extracted an alkaloid named Stephanin from the stems and roots of this plant and reported it at the General Meeting of the Pharmaceutical Society of Japan in April 1923. We obtained crude Stephanin from Mr. Kubomi and purified it, isolating several alkaloidal fractions: an ether-insoluble brown amorphous powder (A) and an ether-soluble fraction (B). Fraction B was further separated into three crystalline alkaloids: a phenolic alkaloid (melting point 232°C), an alkaloid with a melting point of 229°C (presumably Metaphanin), and an alkaloid with a melting point of 152°C (presumably Stephanin). Our study focused on Metaphanin. Elemental analysis and boiling point measurements in chloroform confirmed its molecular formula as C₁₈H₂₉NO₃ (molecular weight ~304). The hydrochloride salt was identified as C₁₈H₂₉NO₃·HCl·H₂O through analyses of carbon, hydrogen, nitrogen, chlorine, and water of crystallization. Structural investigations revealed Metaphanin contains two methoxy groups (OCH₃, via Zeisel’s method) and one N-methyl group (N·CH₃, via Hertzschig and Hans-Meyer’s method), classifying it as a tertiary amine (no reaction with nitrous acid). The remaining oxygen atom was confirmed to be a hydroxyl group by forming an acetyl derivative (melting point 150°C) with acetic anhydride. Comparison of its hydrocarbon core (C₁₅H₁₉) with related alkaloids (Sinomenin, Diversin, Trilobin) suggested it lacks the phenanthrene-based morphine-type mother nucleus due to its fewer carbon atoms.