This study aimed to clarify the effects of a lifestyle intervention by the concurrent use of an activity monitor and Twitter on daily physical activity. Twenty-one healthy males and females (37 ± 13 yrs) were randomly assigned to Normal intervention group (N group, n = 10) or Twitter intervention group (T group, n = 11). In both groups, the intervention period was 6 weeks. Participants in N group wore an activity monitor (Lifecorder EX). Meanwhile, participants in T group were asked to tweet about their daily steps or physical activity in addition to wearing an activity monitor, and also an observer read through the tweet from each participant and commented about physical activity. There were no significant differences in daily physical activity (i.e., steps and amount of physical activity (PA)) at week 1 between both groups. In addition, no significant time-course changes in steps and amount of PA were observed in N group. In contrast, steps in T group were gradually increased from week 1 8,542 ± 3,158 steps/day to week 6 12,700 ± 3,935 steps/day (P < 0.01). Amount of PA in T group was also gradually increased from week 1 2.5 ± 1.2 METs·hour/day to week 6 4.6 ± 2.3 METs·hour/day (P < 0.01). Therefore, these findings indicate that the lifestyle intervention by the concurrent use of an activity monitor and Twitter could effectively induce an increase in daily physical activity compared with the intervention using only an activity monitor.