This research deals with the nonlinear interactions between unsteady fluid flow and vortex-induced vibration of a spring-supported circular cylinder in a narrow channel. The analytical model consists of two-dimensional incompressible viscous flow in a narrow channel at low Reynolds number, and the spring-supported cylinder that can move perpendicular to the wall of the channel. It is considered that synchronized measurement of unsteady fluid flow and motion of a body is very important when the nonlinear interaction between them is discussed.