Fujitsu was in the process of replacing 51,000 Sony-made battery packs in addition to 287,000 already recalled.
Nearly 9.6 million Sony-made lithium-ion batteries are being recalled by a host of computer markers. It was the first such incident in Japan. US computer makers Dell and Apple and China's Lenavo have also reported overheating incidents involving Sony batteries.
Fujitsu and Sony jointly investigated the latest incident and "reached a common understanding" that the battery pack itself was the cause of the heat and sparks, Fujitsu said in a statement.
"At the moment, we are investigating the cause in detail," it said. "We considered the incident to be an extremely rare phenomenon."