A detailed discussion based upon Oseen's equations of motion has been made of the steady two-dimensional flow of a viscous fluid passing perpendicularly through an infinite row of equal parallel circular cylinders regularly spaced. It has thus been found that the drag acting on any one of the cylinders in the row is always greater than that acting on the same cylinder when it is immersed alone in an unlimited uniform flow with the same velocity. In particular, when the Reynolds number of the flow is sufficiently small, the drag is found to be proportional to the flow speed U, while it is proportional to U/log U in the case of a single cylinder. Thus, the interference effect between the cylinders in the row is very remarkable at low Reynolds numbers.