Yumiko Takahashi was accused of parental neglect of her son, Neo, who died of pneumonia June 26, the Mainichi Shimbun reported Friday.
Police said Takahashi, 29, failed to seek medical treatment for her son, didn't change his diapers and left him unattended on a bed in their apartment until the boy succumbed to pneumonia.
Takahashi told investigators the accusations against her were accurate, the Mainichi Shimbun said.
Police said an employee at the nursery school the child attended noticed the boy had a fever when picking him up Sunday. When checking on the child's condition later, the employee said ,Takahashi reported the fever had fallen.
On Wednesday, Takahashi called the nursery, saying, "Something really serious is happening," police said.
When a childcare worker arrived at the apartment, the boy was dead, police said.
An autopsy showed the child had died a day earlier. Police said they believe Takahashi may have been chatting online and doing other things even after her son's death.
Otsu Municipal Government officials said Takahashi's 6-year-old son was placed in foster care after his brother's death.
Police said it was likely that Takahashi had been neglecting her children for some time, the Mainichi Shimbun said. Data on three computers found in the apartment indicated Takahashi spent hours in chat rooms daily.