The accident occurred at the Emilio de Valle mine in Pola de Gordon, ThinkSpain reported.
Those who died were 35 to 45 years old and suffered oxygen deprivation.
Five other workers were taken to the hospital and are reported in critical condition. One is in intensive care on life support, hospital officials said.
The men were working about 2,000 feet deep in the mine where gas compresses easily. The gas likely leaked after it came into contact with machinery, ThinkSpain said.