Developing and cultivating mindfulness exerts a positive effect on psychological and cognitive performance. Sharpening the skill requires continuous mindfulness-based training (MT), which can be challenging for people leading busy lives. Therefore, the current study examined whether trait mindfulness can be improved by a flexible and brief MT program of self-breathing using a pre-post intervention design. Trait mindfulness was assessed using the Japanese version of the Five Facet Mindfulness Questionnaire (FFMQ) before the intervention (pre), after 2 weeks (during), and 4 weeks after the intervention started (post). Data were analyzed using the Friedman test followed by the Dunn-Bonferroni correction. The study sample consisted of 22 healthy participants aged from 20 to 60 years with no previous experience with yoga or meditation equivalent to MT. The mean number of days of MT practice was 26.4, and 11 participants had interruptions. The median values of pre-, during-, and post-total FFMQ scores were 115.5, 123, and 129, respectively. Significant differences were observed in the total pre and post (p < 0.001) and during and post (p = 0.002) FFMQ scores, though a medium effect was found (r = 0.30) only between the pre and post scores. Of the five sub-scales of FFMQ, significant differences were observed only between pre and post Observing (p = 0.01), Nonreactivity (p < 0.001), and Describing (p = 0.01), and during and post Nonjudging (p = 0.016), and Nonreactivity (p = 0.025). Our findings suggest that the simple, brief, and flexible self-breathing method employed in this study has a substantial effect on fostering trait mindfulness and, therefore, can be adopted by people with hectic daily schedules.