In July 1946, Yokoyama was arrested by the US Occupation authorities and was sentenced to death due to command responsibility for vivisection and other human medical experiments performed at the Kyushu Imperial University on downed Allied airmen.
Pada bulan Juli 1946, ia ditangkap oleh pihak berwenang AS dan dijatuhi hukuman mati karena bertanggung jawab memberikan perintah pembedahan makhluk hidup dan eksperimen medis manusia lainnya yang dilakukan di Universitas Kekaisaran Kyushu terhadap para pilot Sekutu yang jatuh dan tertangkap.