To provide coals increased in combustion calorie, which is inexpensive and does not generate environmental pollution, and which is obtained by improving the composition of coals having much moisture and weak thermal power, such as peats and brown coals.

A raw material of coals is heated in a predetermined temperature range in an oxygen-free atmosphere while an inert gas is continuously injected to thermally decompose and individually isolate components other than carbon in the atmosphere and in the coals successively from the components having low decomposition temperatures at a temperature of ≤450°C, thereby obtaining improved coals. Since the carbon constituting the improved coals have high activity, the improved coals have a quality equal to or higher than usual-quality coals, and exhibit more remarkably improved combustion efficiency than the coals when being the raw material.