The 22 foreign tourists and a second guide had a lucky escape from the torrent, without any serious injuries, reports the Bangkok Post.
The cave, next to the Rajjaprabha dam reservoir in Phanom district, is closed to the public from June to November each year, the wet season, because of the danger of flooding.
It was not known why the guides apparently ignored this and took the tourists inside yesterday.
A flash flood poured into the cave about 7:30pm as the group of 22 tourists and two guides were walking out of it.
One of the guides was washed off his feet and carried back into the cave. The others in the party were able to resist the current and were led to safety by rangers. Two Dutch women received minor leg injuries and were treated at a local hospital.
Rangers assisted by rescue workers started a search for the missing guide, Pongyot Koeddee, a 37-year-old native of Ubon Ratchathani. His dead body was found inside the cave about 3:20am this morning.
Nam Talu cave is 600 metres long and 30 metres wide at its mouth. It is best known for its beautiful rock formations, stalactites and stalagmites. It is on the far side from the reservoir pier, which is also used by sightseeing boats.
On Oct 13, 2007, a flash flood killed eight Thai and foreign people who were trapped in the cave.