This paper discusses the design of the mechanism for an artificial mastication device that reproduces the formation of the food bolus by humans. A novel mechanism, the enclosure area expansion and reduction mechanism, that can gather fragments of food is proposed. In the proposed mechanism, multiple movable walls with common shape and sizes form a symmetrical enclosure area that contains the food fragments. Driven by a single actuator, all walls move coordinately so that the enclosure area reduces and expands for gathering the fragments. We first show the mathematical formulation of the geometrical conditions of the movable walls. We then design the movable walls and power transmission system for a prototype installed in the artificial mastication device. Finally, we present the experimental results, which validate the proposed method.