When the Japanese occupied the Indies in February 1942, Nio was one of at least 542 ethnic Chinese from Java and Madura who were arrested and detained. He was held in Bukit Duri, then Serang, and then Cimahi, before ultimately being released in 1945 after the Japanese surrender and Indonesian proclamation of independence. A written account of his experiences while an internee was published in 1946, with the title "Dalem Tawanan Djepang". Sinologist Myra Sidharta describes it as valuable account of history, as other former prisoners did not write such detailed memoirs.