There are not any evidence that sex slave were exist in Japan's army in ww2. The word "sex slave" was made recently for Korean's harassment. Sex slave have been exist from ancient era in Korea. The poor people sold their daughter as a "sex slave". In the ww2 era Korea was very very poor so many parents sold their daughter to Korean trader then. After the WW2 because of Korean Confucianism teaching the Sex slaves could not blame their parents the Sex slaves changed responsivility of being sold by parents or Korean trador on Japan.