Mothers. Japan is actually known for having very poor child support options outside of family. The percent of married woman with children that work is quite low.
According to a government survey from 2017 or so, the number of single parent families was hovering around 1.2 million. This is almost exactly 1 percent of the population which doesn't seem much until you compare it to the number of children under the age of 14 which is 15.53 million people.
Now their ratio gets closer to 8 percent.