It was not immediately clear whether the sword was real or a mock weapon. Police said Tuesday they were investigating whether the death of 33-year-old Daigo Kashino was criminal or an accident.
Kashino, a theatre group actor, had been rehearsing with several others at a studio in Tokyo at the time of the accident on Monday, broadcaster NHK reported.
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He was rushed to hospital but died soon afterwards.
Nobody saw exactly how Kashino was stabbed, according to local media.
Several actors said they had turned around and seen Kashino hunched up after hearing him groan, according to NHK.
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