The 43-year-old was killed in an incident at Ballacrye on the north of the island and the qualifying session was immediately red flagged, the event's organising body ACU Events said.
Yoshinari, from Saitama, had competed at the TT since 2009, finishing fifth in the 2011 TT Zero for electric bikes as well as racing in the superbike and superstock classes.
He was also 2008 Motegi Endurance race winner.
A statement on the ACU website read: "The ACU wishes to pass on their deepest sympathy to Yoshinari's family and friends."
Gary Thompson, MBE, Clerk of the Course, ACU, commented: "Yoshi was a really popular competitor who had a large number of friends in the TT Races paddock.
"He was a genuine and friendly character who always had time for everyone. He will be sorely missed."
The Coroner of Inquests has been informed and an investigation into the circumstances of the accident is under way, added the statement.