Gitelman's syndrome (GS), an inherited disorder due to loss of function of ion channels and transporters such as Na-Cl co-transporter (NCCT) in distal convoluted tubules, is characterized by hypokalemia, hypomagnesemia, hypocalciuria, metabolic alkalosis and hyperreninemic-hyperaldosteronism. A 39-year-old man was admitted to our hospital because of muscle weakness with such intractable disorders. We performed a thiazide-loading test, which revealed a poor response of the fractional excretion rate of chloride compared to healthy subjects. Based on these data, the clinical diagnosis of GS was made. Gene-sequencing analysis revealed compound heterozygous mutations of c.539C > A and c.1844C > T in SLC12A3, which is newly reported in Japanese GS.