The national police agency filed the request with the internet industry, telecommunications companies and cable broadcasters on Wednesday, saying "the risk is high for third parties to inhale the gas and, in worst case scenarios, die".
But Hiroya Masuda, Japan's internal affairs minister, told reporters that the government would not force websites to co-operate with the request, saying such a move would constitute "excessive restrictions on freedom of expression".
In the latest case reported in Japanese media on Thursday, hundreds of people were forced to evacuate their homes amid a cloud of toxic gas caused by the suicide of a 24-year-old man.
|Japan's government says it will not force |
internet providers to co-operate [EPA]
The man's 58-year-old mother was found lying unconscious nearby.
Both were rushed to hospital where the son was confirmed dead.
A police official said the mother was recovering and that hydrogen sulphide had been detected at the scene.
He said 350 residents in the neighbourhood were told to evacuate, "given the past cases in which the gas injured others", before they were allowed to return a few hours later.
Japan, already with one of the world's highest suicide rates, has seen suicides using the gas almost every day in the past month.
At least 48 cases of suicides using the gas were reported throughout the country from March 27 to April 29, according to emergency services.
Many victims post a note to warn neighbours or rescuers about the deadly gas, apparently part of instructions found on websites.