The incident took place at Kachari Sawanga village, about 65 km from here.
A local, Ravi Jaiswal, had engaged the labourers to desilt his well to get more drinking water.
Initially, two of them entered the well and fell unconscious after inhaling poisonous gas. Later, four other workers entered the water body but all of them lost their lives, police said.
The bodies have been fished out and a probe was on, they said.