Yoshioka was born in Fukuwatari-cho (in present day Okayama City), Mimasaka Province as the third son of Yurin YOSHIOKA, a physician (who studied Western medicine in Nagasaki and became a famous obstetrician) and 'Towa.'
吉岡は美作国福渡町（後の岡山市内）の医家・吉岡有隣（長崎で蘭方医学を学び産科医として名を挙げていた）と「とわ」の三男として生まれた。