A horizontal rotating cylinder reactor with a new simple internal spiral transport mechanism was proposed for its application to biomass gasification. In this rotary reactor, the gasification chamber and the combustion chamber are separated by the spiral transport mechanism. And, solid particles move between two chambers through the transport mechanism. However, the gases in the chambers hardly mix because the transport mechanism is filled with solid particles, which act as gas seal. By applying this to a gasifier, high/middle calorie fuel gas is expected to be produced by using air without indirect heating. In the present study, a basic examination on particle behavior was carried out by using the cold model to develop a new rotary kiln gasification process. As the results, the followings were elucidated. The transfer rate of the particles as heat charrier through the mechanism is increased with the increased exit diameter of the mechanism, while, it is necessary to reduce the ratio of the exit size to the entrance size and have a flow path with a constant width of at least 1 turn to form the gas seal. Therefore, it is desirable to design the exit diameter widely within the range where the gas seal can be formed.