Falls and fall-related femoral neck fractures are severe problems for the elderly. In this research, we assumed that people have their own PD controller to keep their balance while standing still. We proposed a screening test in which people shake their heads, conducted balance ability comparison between before / after shaking a head using the PD controller parameters based on CoM-ZMP model and using CoP area size. As a result, it is clarified that people adjust their gains of the PD controller to react the disturbances as shaking their heads. Especially, the differential gain parameters were almost twice. This makes the human balance controller less robust to another disturbance after head shaking, and it might support our hypothesis that the elderly tends to fall when they turn many times in a short time.