Four workers fell into a manhole in Gyoda City, Saitama Prefecture, north of Tokyo on Saturday morning. Police say all of them have been confirmed dead.
The four men, all in their 50s, were inspecting a sewage pipe when the accident occurred.
They were later rescued, but were unconscious and rushed to hospital.
Police say the manhole is 60 centimeters in diameter and more than 10 meters in depth.
They say it is believed that one worker fell into the hole and was followed by the three others who tried to save him.
City officials say the workers were conducting an emergency inspection of sewage pipes that the central government had ordered municipalities to carry out after a massive road cave-in occurred in Yashio City, Saitama Prefecture, in January.
AloJapan.com