Mid-air imaging is promising for glasses-free mixed reality systems in which the viewer does not need to wear any special equipment. However, it is difficult to install an optical system in a public space because conventional designs expose the optical components and disrupt the field of view. In this paper, we propose an optical system that can form mid-air images in front of building walls. This is achieved by installing optical components overhead. It displays a vertical mid-air image that is easily visible in front of the viewer. Our contributions are formulating its field of view, measuring the luminance of mid-air images formed with our system by using a luminance meter, psychophysical experiments to measure the resolution of the mid-air images formed with our system, and selection of suitable materials for our system. As a result, we found that highly reflective materials, such as touch display, transparent acrylic board, and white porcelain tile, are not suitable for our system to form visible mid-air images. We prototyped two applications for proof of concept and observed user reactions to evaluate our system.