The present invention provides C-glycoside derivatives and salts thereof, wherein B ring is bonded to A ring via --X-- and A ring is directly bonded to the glucose residue, and it is usable as a Na.sup.+-glucose cotransporter inhibitor, especially for a therapeutic and/or preventive agent for diabetes such as insulin-dependent diabetes (type 1 diabetes) and insulin-independent diabetes (type 2 diabetes), as well as diabetes related diseases such as an insulin-resistant diseases and obesity.