Ouen-Kaigo is one of the last diaper which have plastic back in Japan from Hakujuji corp. This is a kind of low-end model, so price and performance like absorbency are lower than other products.
According to Hakujuji, this product absorb about 120ml*3=360ml(12.17oz).

Disappointingly, they "improved" to cloth-like back in March 2012.