Yuki Muto, 21, from Tokyo's Shibuya-ku, told officers that he disposed of the breasts by putting them in the garbage disposal in his kitchen sink.
Muto is under arrest for slicing the body of his 20-year-old sister Azumi into more than 10 separate parts, mostly at the joints. He put the other body parts into four plastic bags and hid them in his room.
"I was planning to get rid of (the bags) later," Muto was quoted as telling police.
Muto also admitted to Azumi's murder. "I hit her on the head and she passed out, but then she came round again," Muto told officers. "So I then strangled her."
The victim's lungs contained water, and police suspect that Muto might have strangled or drowned her in a bathtub. (Mainichi)