The incident caused a diplomatic row between the countries
China has said that frozen dumplings which made 10 people ill in Japan also caused illness in China, Japanese media have reported.
Both countries had denied that the contamination with pesticides happened on their territory but they had cooperated on investigations.
The statement is being seen as a near admission that the contamination happened in China.
The issue will be discussed during top level meetings in Beijing on Friday.
Japanese news agency Kyodo quoted the Chinese foreign ministry as saying that the Chinese government placed "great importance" on the incident.
"The public security authorities are currently investigating with their utmost effort," the statement said.
The row over who was responsible for contaminating the frozen dumplings caused a diplomatic rift between the two countries this year and generated intense media coverage in Japan.
Ten people from three families became ill - a five-year-old girl seriously - after eating the imported product, made by the Tianyang Food company in China's Hebei province.
At the time, a Chinese official said there was "little chance" that the pesticide, identified to be methamidophos, had entered the product in China.
But in response to questions from the Japanese media on Wednesday, the Chinese foreign ministry said that there had been cases of food poisoning in China caused by the same chemical found in the same make of dumplings.
The BBC's Chris Hogg in Japan said that many people see the announcement as proof of what they had long suspected - that the incident was another example of poor Chinese food safety standards and not a Japanese problem.
Prime Minister Yasuo Fukuda is expected to use a planned meeting with Chinese President Hu Jintao in Beijing on Friday to address the issue.
"I think Japan and China will further cooperate on this matter to find out the truth as soon as possible," said government spokesman Nobutaka Machimura.
The incident was one of several alerts over the safety of Chinese-made products, including poisoned pet food and concerns over dangerous toys.