On January 20, 1915, Inada and Ido announced the discovery of the causative agent of Weil's disease. Subsequently, on February 13, 1915, they published the first paper on the discovery of the causative organism (a new species of Spirochaeta) of Weil's disease. Besides discovering the causative organism of the disease, Inada and colleagues clarified the pure culture in medium, and determined the source and route of the infection, its pathology and morbid anatomy; the distribution of the organism in various organs and tissues; the excretion of the spirochete, and its division, filterability, and morphological characteristics; and the clinical picture, laboratory findings, diagnosis, prophylaxis, and treatment of the disease. These studies were conducted by Inada, Ido, Kaneko, Hoki, and Ito, in the years 1914 to 1915. In the early investigation of leptospirosis, Inada and colleagues played a prominent part. We would like to remember these remarkably complete and definitive original achievements on leptospirosis made by Inada and colleagues.