The number of elderly people with declined physical function has increased, and supporting independent living for them is important. This study focuses on sit-to-stand motion since it is an important daily activity. The assistive robot is needed to assist the elderly’s sitto-stand motion without interference in the motion. Meanwhile, the physical therapists improve muscle activity of hemiplegic patient by teaching the patient the timing of motions. Thus, this study proposes the assistive chair using skills of the physical therapist for the sit-to-stand motion to support. The seat of the chair rises when the user starts bending forward and stimulates the hip muscles. The three healthy elderly people were measured with and without the assist by the chair. Here we show that the assistive chair was able to estimate the timing when the upper body of subjects started to bend forward. Furthermore, we found that the assistive chair intervention reduced the amount of forward displacement of the elderly’s center of mass. These results imply that the assistive chair teaches the timing of switching the center of mass from forward to upward without pushing too much and improves the sit-to-stand motion of the elderly.