A simple method for improving the quality of electronic portal imaging device (EPID) portal images was proposed for the reduction of the burden on the registration between digital reconstruction radiography (DRR) and EPID portal images in radiation therapy. Conventional image filtering techniques in the spatial-frequency domain are applied to the proposed method. While a band-pass filter (BPF) is employed to extract spatial-frequency components included in the bone edge, a high-pass filter (HPF) is employed to obtain the effect corresponding to the general dynamic range compression. The band-pass filtered image is weighted by a parameter for adjusting the bone edge enhancement, and is added to the high-pass filtered image. This method was applied to the portal images in the neck region. In the image obtained by the proposed filtering, the bone edge was clearly observed. In addition, soft tissue structures were identified in the same display settings (window level/width; WL/WW) as the bone edge observation; that is, the adjustment of the display settings was not required for the observation of each object. These results suggested that both bone edge enhancement and dynamic range compression would be achieved successfully. It was estimated that the images obtained by the proposed method were more appropriate for the registration than conventional portal images, in 47 times registrations of 50 times in total (the registrations by five radiological technologists in ten patients). The proposed method was concluded to be useful for improving the quality of portal images, enabling the efficient registration.