This report deals with a simplified method for calculating maneuvering hydrodynamic forces on a ship taking off-centerline course in a narrow water channel. In this report, calculation is made by using infinite images with respect to the side walls and the water bottom. The ship body is replaced by sources and vortices distributed in the center plane. The source strength is proportional to the slope of a distribution curve of ship width. The hydrodynamic forces are evaluated on the basis of the linear lifting surface theory. Numerical calculation is made by changing channel width, water depth and off-centerline displacement systematically. Calculated values are compared with the experimental results presented by Prof.Fujino. As a result, it is indicated that the calculated values agree qualitatively with the experimental results. It is clarified that the value of the correction factor, which is introduced to provide effective width of the channel, is almost constant regardless of ship form, channel width and water depth.