Highly potassic mafic rocks are rarely found in Southwest Japan Arc. Recently, we found highly potassic lamprophyre (minette) from the northeastern part of the Sera Plateau in the Chugoku Mountains. It consists of phenocrysts of phlogopite (mg 85.6, TiO₂ 4.08-4.18), clinopyroxene (diopside-salite), olivine (mg 84.5) and apatite set in a groundmass of poikilitic sanidine ?? anorthoclase, phlogopite, clinopyroxene, apatite and titanomagnetite. The minette is high in K₂O content (5.88 wt%) and K₂O/Na₂O ratio (3.51). The rock shows relatively high concentration of MgO, Ni and Cr, and is strikingly enriched in incompatible elements, particularly Rb, Sr, Ba, Zr and P. The MORB-normalized pattern of the minette is most closely matched by certain alkali basalts of island arcs and continental margins, having negative Nb-anomalies. However, it is highly enriched in incompatible elements compared with island arc basalts. This suggests that the source for the minette is similar to that for the island arc alkali basalt but spiked with K, Rb, Sr, Ba, Zr and P prior to melting.