Recently we conducted investigations in biological wastes because large pile-up storage of waste wood chips and others caused many fires in Japan. This paper shows the experimental results on wood chips with thermal analysis, by using a Thermogravimetry-differential thermal analysis (TG-DTA) and micro calorimeters, and as well with spontaneous ignition measurements, such as a UN wire mesh cube tester and a spontaneous ignition tester (SIT). Exothermic reaction of wood chips was observed during 45-60 degrees C only by the high sensitive microcalorimeters, TAM and MS 80. This reaction is far apart from the second major reaction by oxidation and is not easy to be recognized by the conventional detectors, like the TG-DTA and the wire mesh cube tester, because their sensitivity cannot meet the strict requirement. Correspondingly, experimental results under the adiabatic condition in the SIT confirmed this theory, in which the onset temperature of spontaneous ignition of wood chips was measured as 50-80 degrees C. This implies that the weak initial reaction at ambient temperatures mainly results from microbial fermentation in the presence of its inherent moisture and possibly gives rise to the further intense combustion sustained by a chemical reaction if the heat cannot be removed from the large scale storage of wood chips.