A novel design method of ridge filters (RFs) has been developed for general proton beam lines which use a single-radius beam wobbling method. It can be applied to beam lines that transport both protons and carbon ions which are about three times longer than regular beam lines dedicated to protons. We designed an RF with an SOBP (spread-out Bragg peak) width of 60 mm in water for the 160-MeV proton beam of the HIMAC (Heavy Ion Medical Accelerator in Chiba) biology beam line using an existing model of the RF. Yet we observed a slope in the SOBP region when we used the RF. To elucidate the source of the slope, we have developed a new calculation model taking into account the geometry of the RF and a beam-limiting device. The source for the slope was found to be the large scattering effect of protons in the RF and beam restriction by a ring collimator (aperture diameter: 160 mm) placed just before the RF. When both fluence reduction by the scattering effect of protons in the RF and the beam-collimation effect are taken into account, proper RFs can be designed universally for a given beam line arrangement using the single-radius beam-wobbling method from the start without any trial-and-error process. This will serve to reduce the commissioning time of newly designed beam delivery systems.