This study aimed to identify 1) the prevalence of "clinical dyspnea," defined here as dyspnea interfering with any daily life activities, 2) the impact of dyspnea on daily life activities, and 3) the screening ability of the Cancer Dyspnea Scale (CDS) and the Dyspnea Numeric Scale (DNS). A total of 157 outpatients with advanced lung cancer completed the two scales (CDS and DNS) along with a questionnaire about interference with daily life activities (normal work, walking, sleep, mood, relation with other people, enjoyment of life, and general activities). Over half of this population (55%) experienced "clinical dyspnea." Dyspnea interfered with not only physical domain (52%), such as walking and work, but also with psychological domain (23%), such as mood and enjoyment. Both scales were feasible for screening of clinical dyspnea. Applying a screening protocol may contribute to avoiding underestimation of clinical dyspnea and lead to appropriate interventions for it.