A clean room has recently become indispensable equipment for various kinds of industries. In order to design a clean room, it is important to consider improvement of the working environment and energy conservation. The air flow of a clean room is very complex. It consists of recirculation flows, and the direction of mean air flow is changed locally. A method of numerical simulation have been developed to delineate a guideline for ventilation design of a clean room. This paper presents an improved finite element analysis for solving unsteady incompressible Navier-Stokes equations. This scheme is based on the algorithm of the HSMAC (Highly simplified marker and cell) method in SOLA and the general idea of orthogonal decomposition. It has actually been applied to the case of the lid-riven square cavity flow at Re=10 000, and of air flow in a conventional clean room. Unsteady flow simulations in terms of stream lines, equipressure lines, and equivorticity lines are obtained on a color graphic display.