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To evaluate in-situ heat release rate of the smoldering combustion, temperature histories in a wood sawdust layer were measured. There were three distinct regions in a measured temperature history. They were attributed to the preheat region, the charring region, and the decaying region. Boundaries between those regions were identified on the temperature history as points of inflection. The smoldering front was considered to be the boundary between the preheat and the charring regions. The fact that the temperature history has a point of inflection at the smoldering front means heat release at the front. The heat release rate at the front if incoming and outgoing heat fluxes at the front were evaluated. To evaluate these fluxes, temperature profile in the layer was derived from the temperature histories at several heights and spread rate of the smoldering front, which was constant during a test run. Heat release rate at the smoldering front was estimated from the difference between the incoming and outgoing heat fluxes at the smoldering front. The evaluated heat release rate showed no dependence on the atmospheric oxygen concentration. From the result, the reaction occurred at the smoldering front was considered to be a pyrolysis reaction. However, the activation energy evaluated from the heat release rate was 13.7 kJ/mol which is much smaller than usually referred activation energy for wood pyrolysis. Oxygen in atmosphere is considered to work, at least, as a catalyst of the reaction. KEYWORDS: Smoldering, heat release rate, wood, temperature profiles.