Recently in Japan, many computer-assisted systems to motivate people to continue walking exercise have been used for health promotion. Some of them try to motivate people using the mechanisms of SNS; they expect that the comradery and the communications among users in SNS systems motivate users to continue their walking exercise. Those systems, however, have a limitation that only the users who afford to make text-based conversation can use the systems. Thus in this paper, we designed a SNS based system to support continuous walking exercise using the number of daily walking steps instead of text-based conversation. Our system is designed in consideration of three motivation levels with three motivation mechanisms to provide various motivation effects without enforcing users so much labor. Through our experiment it is shown that the computer-assisted systems can provide proper motivation effects to have users continue their walking exercise.