A clogging behavior of a high-efficiency particulate air (HEPA) filter has been investigated for solvent fire accidents to provide valuable data for establishing a risk assessment method for reprocessing facilities in Japan. In this study, the burning rates of 30% tri-butyl phosphate (TBP)/dodecane and dodecane solvents and the differential pressure ΔP change of a high airflow–type HEPA filter applied in ventilation systems of reprocessing facilities in Japan were measured in the solvent burning. It was confirmed that the dodecane in the 30% TBP/dodecane mixed solvent burned mainly at the early stage of the burning of the mixed solvent and that the TBP burned mainly at the late stage of the burning of the mixed solvent. In addition, the burning rate of dodecane in the early stage and the rate of the TBP in the late stage were estimated, respectively. As a result, the former rate was almost the same as the burning rate of burning only the dodecane without TBP. Furthermore, the rapid increase of the ΔP of the HEPA filter was observed at the late stage of burning the mixed solvent. The increase of the release ratio of the airborne particles of unburned solvent (i.e., TBP and/or degradation products of TBP) and inorganic phosphorus (i.e., P2O5) was considered to contribute to the rapid increase. The empirical formulas for representing the relationship between the mass of the loading airborne particles onto the HEPA filter and the ΔP of the HEPA filter, except for the region of the rapid increase of the ΔP, under the mixed-solvent burning could be induced.