Japan says China on Tuesday admitted the mystery submarine was one of its own, and expressed regrets.
Japan's Foreign Ministry says the message was conveyed by Chinese Vice Foreign Minister Wu Dawei to Japanese Ambassador Koreshige Anami in Beijing.
Tokyo says Beijing told it the submarine was on a training mission, and for "technical reasons," it ventured into Japanese waters.
Foreign Ministry Assistant Press Secretary Akira Chiba says China's official response also contained an apology.
"We take it as an official apology since the word 'sorry' was used. On the other hand, we have asked them to prevent such incidents from happening again and for that we have haven't gotten any response yet," he said.
China's Foreign Ministry on Tuesday was more circumspect about the matter. At a regular briefing in Beijing, Foreign Ministry spokeswoman Zhang Qiyue would only say that China had passed on information to Tokyo.
Ms. Zhang says China has already informed Japan on the relevant matter through diplomatic channels, and the issue has been properly addressed.
When pressed by reporters for more details, Ms. Zhang merely repeated the same formula over and over.
The submarine, believed to be a Han-class nuclear powered vessel, spent two hours submerged in Japanese waters, near Taiwan, last Wednesday.
The incursion prompted Japan's maritime forces to go on alert for only the second time since the end of World War II.