C-phenyl 1-thioglucitol compounds of the following formula (I) or pharmaceutically acceptable salts thereof or hydrates thereof: [wherein X represents a hydrogen atom or a C1-6 alkyl group, Y represents a C1-6 alkylene group or —O—(CH2)n- (wherein n represents an integer of 1 to 5), and Z represents —CONHRA or —NHCONHRB (provided that when Z represents —NHCONHRB, n is not 1), wherein RA represents a C1-6 alkyl group substituted with 1 to 3 substituents selected from the group consisting of a hydroxyl group and —CONH2, and RB represents a hydrogen atom or a C1-6 alkyl group substituted with 1 to 3 substituents selected from the group consisting of a hydroxyl group and —CONH2] are useful as prophylactic or therapeutic agents for diabetes, because of their suppressive effect on sugar (e.g., glucose) absorption through inhibition of SGLT1 activity, or alternatively, because of their suppressive effect on sugar (e.g., glucose) absorption and excretory effect on urinary sugars through inhibition of both SGLT1 and SGLT2 activities.