This paper describes the fluid force acting on a square cylinder of finite length in the lock-in region. The experiment was carried out in an N.P.L. blow-down type wind-tunnel with a working section of 500mm×500mm×2000mm at Reynolds number of 1.25×104. The cylinder was forced to oscillate sinusoidally in the lift direction. The power spectrum of the fluctuating velocity in the wake behind a square cylinder was measured to show the lock-in region in the present experiment. The time-mean pressure distribution and fluctuating pressure distribution on the square cylinder were measured for the displacement in the vibration. Consequently it was found that the mean drag and fluctuating lift increase and become maximum in the lock-in region, while the base pressure in the rear surface of the cylinder becomes low and attains minimum.
