Masatoshi Koshiba was born on Sept. 19, 1926, in the coastal city of Toyohashi, in central Japan, the second child of Toshio and Hayako Koshiba. His mother died when he was 3, and his father, a military officer, married his deceased wife’s elder sister. They had two sons.
Dr. Koshiba grew up in Yokosuka, a city on Tokyo Bay, and attended an elite high school in Tokyo. He had been thinking of studying German literature at the University of Tokyo until he overheard his physics teacher, who had given him a flunking grade, denigrate his abilities.
“That statement made me furious, so I started studying physics,” Dr. Koshiba said in an oral history interview for the American Institute of Physics in 1997. “After one full month of concentrated work, I passed the physics department requirement, while the favorite student of the professor failed.”
After graduating from the university and two years of graduate study there, he went to the United States and dashed through the University of Rochester, earning his Ph.D. in physics in 1955 after only a year and eight months of study. He had hurried because he wanted the higher salary that a doctorate could command ($400 a month at the time) so that he could have more money to send back to his family in Japan.
After three years as a postdoctoral fellow at the University of Chicago, Dr. Koshiba returned to the University of Tokyo. Shortly after moving back, he entered into a marriage, brokered by physicist friends, with Kyoto Kato, an art museum curator. She survives him along with a son, Shyun; a daughter, Mari Fujii; and two grandchildren.
Dr. Koshiba’s early research involved studying cosmic rays — high-energy particles flying from outer space — by placing detectors on high-altitude balloons. But he was to make his mark in the other direction — namely, underground.