Beijing urged Japan to seriously deal with the accident that left at least 36 people injured -- three seriously, in Qiqihar city after the weapons were discovered on Monday.
According to China's foreign ministry website, the director of the ministry's Asia department Fu Ying called in Japan's ambassador to China on Friday and lodged the protest.
"The use of chemical and biological weapons (constitutes) a severe crime of the Japanese invaders during the Second World War and the deserted Japanese chemical weapons have seriously injured Chinese civilians many times after the war," Fu said.
"Japan shoulders unshirkable responsibility for resolving the problem," she said.
The chemical weapons, discovered at a construction site in Qiqihar, were stored in five barrels, one of which was broken and leaking an oil-like material, Xinhua news agency said.
Workers removed the barrels and sold them for scrap that day, while the leaking barrel also contaminated the area around the dig.
By evening Monday, several people reported headaches, eye aches and vomiting as local officials determined following chemical analysis that the discovered material was mustard gas.
Of the 36 people suffering from exposure to the chemical, 29 were hospitalized.
Doctors further warned that mustard gas also carried a latent effect that could last for months, the report said.
Japan's brutal occupation of Chinese territory before and during the World War II remains a source of constant tensions between the countries with more than 700,000 chemical weapons estimated by Japan to have been abandoned by their retreating armies in the months around their surrender.
Chinese experts say that as many as two million such weapons are still buried giving China the world's largest stockpile of leftover chemical weapons.
Tokyo agreed to fund and organize China's expected billion-dollar clean-up of the weapons when it joined negotiations on the Chemical Weapons Convention (CWC), one of the earliest international treaties aimed at ending the proliferation of weapons of mass destruction.
The convention, signed by China, Japan and 128 other countries in 1993, took effect in 1997.
As well as laying out rules on the non-proliferation of chemical weapons, the pact stipulated that countries which have abandoned chemical weapons in other nations must clean up and destroy them within 10 years of the pact taking effect.