Abstract: An organic substance analyzing method is disclosed. A sample container 21 charged with a granulated rock sample is installed in a thermal cracking furnace 1. The thermal cracking furnace 1 is connected to a vacuum suction line 27 to be evacuated until a substantial vacuum is created therein, whereupon the furnace is closed. The closed thermal cracking furnace is rapidly heated to a first temperature at which hydrocarbons are more or less cracked but inorganic carbonates are not decomposed. The thermal cracking furnace is maintained at this temperature for a fixed time and then is cooled to a temperature at which the hydrocarbons in the thermal cracking furnace do not react with air. Then the furnace is connected to an air supply line 29 and an exhaust line 35 to pass the produced gases in the furnace to a hydrocarbon detector 34 and a CO.sub.2 detector 33. From the output signals therefrom, HC peak P1 and CO.sub.2 peak P3 are found and recorded.