When we investigate the flow around a bluff body such as a circular cylinder, we have to consider hydrodynamic forces induced by an unsteady vortex shedding. The shed vortex sheet is supposed to be deformed three dimensionally or sometimes has an irregular structure, even in case of two dimensional cylinder. It is well known that the lift force (transverse force perpendicular to the oscillating direction) occurs even for a symmetrical circular cylinder and the magnitude of the force is the same order as that of the in-line force. Furthermore it has been confirmed that the lift force varies irregularly. Those characteristics of hydrodynamic forces have not become clear yet. Forces may act on the circular cylinder with a periodic structure called as the correlation length along the direction of an axis, and that under some conditions lift forces are surprisingly varied three dimensionally as the behaviors of lift forces are unperiodic or chaotic. Now it is strongly required how the lift force is estimated for designers of marine structures. We have reported experimental results of hydrodynamic forces acting on a vertical circular cylinder oscillating sinusoidally at low frequencies in the still water, to examine the flow around a circular cylinder, particularly the three dimensionality and the irregularity of lift forces at low Keulegan-Carpenter number ( Kc ), From those results it was shown that three dimensionality of the lift force is remarkable and the value of the correlation length along the direction of an axis might be the same order as a diameter of the cylinder. However, roughness of the cylinder, which means the slits between each test section of six-divided model, has much effect on the flow including three dimensionality. For that reason we could not estimate the value of the correlation length by using the six-divided model, the test section of which consists of six slices of cylinders. In this study, to investigate the flow around a cylinder, especially the three dimensionality of the flow with reducing effects of roughness in more detail, we measured forces with the same methods with four kinds of cylinders. Each cylinder has a value of the length-to-diameter ratio L / D ( L : length of test section, D : diameter= 12 cm) of a test section (one-body model), L / D =1, 2, 3 and 4. The reduction rate of C _Lrms due to three dimensionality was obtained. Furthermore the behavior of the lift force was examined from a viewpoint of chaos. It was shown that the correlation dimension is a good index representing the characteristics of the forces.