The aim of this study was to isolate and characterize minor alkaloids from Sinomenium acutum REHD. et WILS. native to Japan, excluding the already known alkaloids (sinomenine, disinomenine, tuduranine, sinactine, acutumine, acutumidine). The methanol extract of S. acutum was treated with 2% tartaric acid, and the crude alkaloid fraction was sequentially extracted with benzene, chloroform, and water-saturated butanol. The chloroform layer, which contained known acutumine and acutumidine, was excluded. The benzene layer, after removing sinomenine by recrystallization, was acetylated with acetic anhydride-pyridine to separate into neutral (A) and basic (B) substances. The water-saturated butanol layer was concentrated, continuously extracted with chloroform using the Soxhlet method, and the resulting alkaloid and neutral fractions were acetylated to obtain neutral (C, E) and basic (D, F) substances. Isolation was performed using silica gel and alumina column chromatography, and characterization was based on physical constants (melting point, specific rotation), spectral data (UV, IR, NMR), elemental analysis, and comparison with authentic samples. From neutral substance (A), N,O-diacetyl michelalbine (I, mp 241-243°) was isolated, which hydrolyzed to michelalbine (mp 204-206°) matching the authentic standard. Additionally, N-acetylstepharine (II, mp 220-221°) was identified from (A) by comparison with the authentic sample. From neutral substances (C) and (E), two new minor alkaloids were obtained: III (C₂₃H₂₅O₆N, mp 194-195°) and IV (C₂₇H₃₃O₁₁N, mp 94-97°), whose structures are under investigation. Basic substance (B) mainly contained acetyl sinomenine (mp 175-179°) with three additional TLC spots. Basic substance (D) primarily contained sinomenine with three spots, while (F) remained unstudied. In conclusion, minor alkaloids including michelalbine, stepharine, and two new alkaloids were successfully isolated from S. acutum.