We have developed a method for combining the images of multiple flat-panel displays to improve the quality of integral three-dimensional (3D) images. A multi-image combining optical system (MICOS) is used to magnify and combine the images of multiple displays without gaps between multiple active display areas. However, in the previous prototype, the image quality of the 3D images deteriorated due to the use of a MICOS that had a complicated structure and a diffuser plate. This paper describes an optical system for combining multiple images while suppressing the deterioration of 3D image quality. The improved method can suppress the deterioration of the image quality because it uses a simple structure as a MICOS and does not require a diffuser plate. Furthermore, the thickness of the entire equipment was increased because parallel light was required for the backlight of the LCD panel in the previous design. The thickness of the entire equipment could be reduced to 1/5 or less because diffused light can be used in the improved design.