The man, who is a New York resident, was arrested last week as police were investigating the disappearance of a 27-year-old woman from Hyogo, reported missing by her family on February 17.
The Japanese woman was spotted on surveillance cameras entering the condominium together with the American man just hours before she was missing. He was later seen going in and out alone several times, carrying a large bag, but the woman was never seen leaving the building.
Shortly before her disappearance, the woman had told her friends she was going to meet with a US man she had met via an app. She had also written on Instagram that she was going to "meet Jay".
Body parts other than the head have not yet been found. The suspect had arrived as a tourist in Osaka in January. He has denied the allegations, as reported by the Japan Times.