The man told a packed Tokyo press conference late yesterday that he was shell-shocked when he learned the truth, saying his life would have been starkly different.
"I've wondered 'how on earth could this happen'? I couldn't believe it. To be honest, I didn't want to accept it," he told Japanese media late yesterday. The man was not identified.
"I might have had a different life. I want (the hospital) to roll back the clock to the day I was born."
It is not clear if the hospital, which has not commented on the bizarre case, will appeal.
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The man, an unmarried truck driver, would have grown up as the eldest of four brothers in a wealthy family where siblings enjoyed a lavish lifestyle including private tutors.
Instead, he was raised on welfare by his non-biological mother who also supported older siblings after her husband passed away.
"It was like she was born to experience hardship," the man said of the woman he knew as his mother.
The man has been helping take care of his non-biological brothers, one of whom suffered a stroke.
The decades-old mistake was uncovered when the wealthy family's three younger brothers had DNA testing done on their oldest sibling - who looked nothing like them - after their parents died.
They checked hospital records and confirmed the identity of their biological eldest brother last year, whom the men have since met.