The relationship between the composition of inorganic components in sewage sludge, their melting points, and the amount of molten material was investigated with differential thermal analysis in order to elucidate the mechanism of the clogging phenomenon caused by adhered materials in the sludge incineration system of a sewage treatment plant and to develop a countermeasure. The sludge in the sewage treatment plant where the clogging phenomenon occurred contained substances with a melting point of around 860°C throughout the year. During the winter season, the melting point of the sewage sludge dropped to nearly 800°C, which is below the sludge combustion temperature, indicating that the presence of low-melting-point compounds plays a significant role in the occurrence of the clogging phenomenon. The melting point decreased and the amount of melt tended to increase as the P₂O₅ content in the sludge increased, indicating that P was an component that promoted the clogging phenomenon. From the results of differential thermal analysis on simulated samples in which reagents were added stepwise to calcined real sewage sludge, it was estimated that the higher the Mg, Al, Ca and Si content and the lower the Na content, the less likely the clogging phenomenon occurred.