Pyrolysis of woody biomass was studied experimentally with the aim of investigating heat transfer and heterogeneous chemical reactions. In rapid pyrolysis, two different pyrolysis rates were obtained, with different reactions taking place depending on the temperature (a process producing gas and a process producing tar + water). However, the temperature at the transit stage between the two steps for gas differs from the temperature for tar + water. The fact means that there are two processes for gas generation. The experimental results obtained for slow pyrolysis showed that the average temperature in the biomass layer increased slowly as compared with the change in the set temperature. This tendency does not vary over a mean particle size range of 1.1 < D-p,D-50 (mean particle size) < 11 mm. Numerical simulation of heat transfer in a tubular reactor, without considering chemical enthalpy, was carried out. Comparison of the calculated results with the experimental heat flow rates in the biomass layer revealed two endothermic regions and two exothermic regions. The flow rate of gas generated in the reaction showed two peaks at the exothermic regions. Thus, it was concluded that heat transfer by pyrolysis consists of many processes. Furthermore, we propose a new mechanism representing the heterogeneous chemical reactions involved in pyrolysis, which is divided into four processes: (i) evaporation of water, (ii) decomposition of cellulose, (iii) decomposition of generated tar and (iv) decomposition of lignin.