Hiroki Yuhara, a former member of the Japan national side and the Toshiba Brave Lupus forwards coach, died at the age of 36 on Monday, the Top League club announced.
The cause of death has not been revealed, although sources with knowledge of the incident said he collapsed while working out in the team's clubhouse in western Tokyo and was rushed to a hospital, where he died.
The former hooker won 22 caps for Japan and featured in one game at Rugby World Cup 2011 in New Zealand.
"This news is so sad and sudden. I have no words," Japan Rugby Football Union President Shigetaka Mori said in a press release.
"His contributions to the national team for so many years as a cornerstone of our pack are immeasurable, and we were counting on his future contributions as a coach."
Full story via KyodoNews