The present study investigated the daily fluctuation of ruminative thinking and its individual differences by using the experience sampling method. Participants recorded their thought contents and negative affect eight times a day for a week at semirandom intervals. High-trait ruminators showed high levels of self-focus, unpleasantness, and uncontrollability in their thoughts over the sampling course. These variables were interacted to predict the levels of concurrent negative affect: Self-focus was strongly associated with increased levels of negative affect when the thought was highly unpleasant and uncontrollable. A composite measure of rumination, including self-focus, unpleasantness, and uncontrollability, exhibited diurnal variation, which was assimilated by a quadratic function of time of day. However, there were differences in the estimated parameters of diurnal trajectories between high and low levels of depression, which indicated that individuals with higher levels of depression are more likely to engage in rumination in the evening, not in the morning, than those with lower levels of depression. These findings suggest that rumination in the evening would play an important role in the exacerbation and maintenance of depression.