I am assuming that the Japanese script in the top left of this Canadian POW to be sent to Japan card reads "Prisoner of War" or something similar. Google translate does not help as I suspect it uses modern Japanese. The answer it produced fro Prisoner of war does not match what is on this card.
The late Ken Ellison's collection of Force "C" covers contained a number of these cards used by Canadian civilians to Prisoners of War held by the Japanese. He translated the Japanese inscription as "Prisoner of War Mail" E. R. Toop gives a bit more information on the card. It was introduced in July 1944 for use to the Far East. Postage was 10c, but was reduced to 5c on 25 July 1944. Finally, in January 1945 the cards were accepted free of postage. There were three printings, identified by the rate "Affix 10c Postage"; the second read "Affix Postage Here" and the third printing read "Postage Free / En Franchise". Your card is from the first printing.
The Ken Ellison reference is to "Force 'C' - The Canadian Army's Hong Kong Story 1941-1945" p. 45; BNAPS Exhibit Series No. 8. The other information came from Ritch Toop's "The Major E. R. Toop Collection of Military Postal History Volume One" p. 367; published by BNAPS. The dates are from Ritch's album page.