Aulacus machaerophorus sp. n. is described from females collected in Sapporo, Hokkaido, Japan. This new species has some interesting characteristics such as a very short ovipositor, absence of a ventral lobe and ovipositor guide on hind coxa, and smooth and shiny axillula, and is considered to be closely related to Nearctic species, A. schiffi. A key to three Japanese species of Aulacus is given.