State media in China said the tainting of the dumplings, which sickened 10 people and caused an international row, was being linked to one food processing worker acting. Lu Yueting was said to have poisoned the dumplings after a dispute over his pay.
Traces of the insecticide methamidophos were found on the packaging and in the vomit of the people who were sickened in December 2007 and January 2008 after eating two separate brands of dumplings produced in northern China's Hebei province.
"Japan will continue its cooperation with China on this matter. We hope to see an early resolution to the case," said Japanese Prime Minister Yukio Hatoyama.
The poisoned dumplings led to a food-safety scare in Japan and damaged Chinese efforts to improve foreign consumer confidence in its exports after a series of food and product safety scandals.
China's food exports to Japan dropped by almost a third after the poisonings. Japan is the third-largest market for Chinese exporters of fish, dumplings and other processed food.