To provide an inside/outside determining method and its program which is robust against incomplete boundary information and can determine inside/outside even if boundary information is incomplete, whose processing time is short and high-speed, which can be installed easily in a computer, does not raise fears that different spaces are classified into a single space and can be applied to a multi-space.

This method has an external data input step for entering an external data 12 made of boundary data of a target 1 (A), a cell division step for dividing the external data to a rectangular solid cell 13 crossed by a boundary plane (B), a cell classification step for classifying each of the divided cells to a boundary cell 13a including the boundary data and a non-boundary cell 13b not including the boundary data (C) and a space classification step for classifying the non-boundary cell 13b to a plurality of spaces partitioned by the boundary cell 13a (D).