The outburst occurred last night when 51 workers were underground at the mine in the city of Dengfeng in Henan province, state broadcaster CCTV said.
Thirty-nine workers rose to safety while the others were trapped or killed, the report said.
Workers were doing maintenance work in a pump room when the roof fell, causing the outburst of gas, according to the official Xinhua News Agency. Some reports said three rescuers were among those who died, but that could not immediately be confirmed.
China's mining industry has long been among the world's deadliest, and top work safety regulators have acknowledged that some mines cut corners on safety standards due to financial pressure.