Megumi Shimamura, 30, of Misato, Saitama Prefecture, blamed her son for the death of his brother in her 11-day absence, according to the Saitama District Court, the Asahi Shimbun reported.
Judge Yujiro Nakatani chided Shimamura, saying 'such a heartless and inhumane crime makes one doubt (your) self-awareness, affection and humanity as a parent'.
Shimamura, who is unemployed, left her house on March 3 because she wanted a break from child-rearing and to hang out with her boyfriend, according to the Asahi.
Her mother apparently gave Shimamura the money she needed to keep herself and her three young children alive, the Mainichi Shimbun reported.
Shimamura reportedly told her six-year-old son: 'I will not return so take care of things.'
He called Shimamura's mobile phone several dozen times a day but was largely ignored, the Asahi said.
Shimamura left hamburgers, bread, cookies and other food at the entrance of the house once or twice a day.
But the food was not enough for the three children, according to the court.
On March 12, the six-year-old called Shimamura to say his brother would not wake up.
She returned to the house but did not have the courage to enter it. Instead, she went to a pub and got drunk.
The next day, her son called her again, saying: 'He still won't wake up.'
On March 14, Shimamura returned home and found the two-year-old boy dead in a crib covered in garbage and faeces, Asahi reported.
She slapped the older boy's face and said: 'You are disqualified as my child. Though I'm responsible, you are also responsible. You must have eaten all the food by yourself.'
The two-year-old starved to death on March 12, according to the court.
The six-year-old told prosecutors: 'Mum told me I was fired because I didn't take care of the twins. It's all my fault.'