After examining the characteristics of the heat decomposition of the 80~120mesh flour using the Mini cup pressure vessel test and determining the apparent activation energy in a spontaneous combustion, the conclusion is as follows. The heat decomposition of flour occurs at around $100^{circ}C$ and the peak for the maximum rise in pressure appears at around $290^{circ}C$. The decomposition pressure against various temperature in the vessel shows the maximum value of $4.7kg/cm^2$ approximately at $440^{circ}C$. When the thickness of the sample is 3cm, the maximum temperature and the critical temperature of ignition are $398^{circ}C$ and $204.5^{circ}C$, respectively; the critical temperature is $194.5^{circ}C$ when the thickness of the sample is 5cm, and $182.5^{circ}C$ when the sample is 7cm. In addition, the apparent velocity calculated using the method of least squares is 35.0407Kcal/mol.