Kusumoto was born in the village of Shichikama, later incorporated into the town of Saikai, in Nishisonogi District, Nagasaki on 10 March 1871, or the 20th day of the first month by the old calendar then still in use.

While in Germany, Kusumoto studied the Zarichin effect of the relationship between bile and fecal hyperlipidemia in regards to diet.

He always made efforts to be courteous while treating the patient, as he had often noted doctors seemed to regard clinical studies as more of a hobby than a true duty.

In September, he established the Research Institute for Microbial Diseases with donations from the business community, and headed a group to study cancer treatment in August 1935.

The study group began cancer treatment research by buying 3g of radium from Czechoslovakia, which was widely reported in newspapers.

He was appointed Director of the Institute of the Japan Society for the Promotion of Disaster Science of Osaka Imperial University in January 1937.