Monocular head-mounted displays (HMDs) will be a major tool for ubiquitous computing. However, monocular HMD visibility causes a decrease in the light received from the environment. Fluctuating light from the environment strongly affects the visibility of images presented on the monocular HMD. In the present study, we have studied a method for controlling HMD backlighting to improve visibility at the wearing a monocular HMD under variable lighting conditions. Three backlight control patterns were developed from Experiment I as psychophysical measurements, and their validities were subjectively evaluated in Experiment II. The results suggest increased visibilities at wearing a monocular HMD when controlling the backlight. Moreover, the rules for backlighting control are based on psychophysical theory.