Although the flow through the poppet valves of the four-stroke cycle engine and piston valves and rotary valves of the two-stroke cycle engine is unsteady, because of the periodic area change of the flow passage of the valves, the flow characteristics of these valves is usually evaluated by steady-flow conditions. The author experimented with flow characteristics under steady and unsteady conditions by using a rotary valve, in order to specify the difference between them and to identify the factors that effect this difference. According to the results obtained, inertia of the mass of the fluid in the flow passage of the valve is the most important factor in creating this difference of flow between them. A theoretical analysis of this inertia was found useful in predicting the effect of various parameters on flow through the passage of the valve under an unsteady condition. These results have brought about a better understanding of the phenomena that occur during unsteady flow through the valves.