Cahnite was found in a vein that intruded crystalline limestone in the vicinity of skarns at Fuka, Okayama Prefecture, Japan. This is the first occurrence of cahnite in Japan. It occurs as aggregates of tetrahedral crystals up to 1 mm long on calcite crystals that grow on cavity walls in the vein, and as anhedral crystals in direct contact with johnbaumite in the vein. The other associate minerals are mainly andradite, arsenopyrite and löllingite. Electron microprobe analyses and ICP-MS give the empirical formula Ca_2.09B_0.95As_0.93Si_0.06O_3.92(OH)_40.8 on the basis of O=8. The unit cell parameters in tetragonal system are a=7.101(1) and c=6.192(1)Å. The mineral is optically uniaxial negative with refractive indices ω=1.658(1) and ε=1.657(1). The Vickers microhardness is 413 kg mm⁻² and the Moh's hardness number is 4.5. The density is 3.13(2) g cm⁻³. It is likely that cahnite at Fuka was formed as a secondary mineral by a late-hydrothermal alteration of johnbaumite.