Draft of the conbustion gab burning in a briquette gives a great part of driving force to the flue gas in an Korean conventional ondol heating system. The Paper present s how the draft intensity varies for each whole Combustion duration. In this experiment the temperatures of thirty spots In three gas courses of a briquette were measured and the drafts were calculated form those experimental data. The result is that the shorter the combustion duration of a briquette is, the stronger the draft becomes, the intensity of drafts ranging between 0.15mm Aq to 0.26mm Aq.