It is very important for designers to achieve safety and usability of products. In this paper, first a gap model between designers and users is proposed as a framework to detect problems in safety and usability and to analyze the causes. Then methods to describe operations to a product, simulate its behavior as state transitions, and detect unsafety gaps and invalidity gaps based on temporal intervals and their qualitative relationships are proposed. The proposed methods are implemented as Gap Analysis Program (GAP) and applied to analyzing operations of a farm tractor by 8 general participants and 4 tractor-manufacturer participants. As the result, potential unsafety gaps and invalidity gaps are appropriately detected
