The purpose of this paper is to investigate non-work relating activities at work. To do this, this paper proposes and analyzes a simple theoretical model consisting of factors such as non-work relating smartphone usage during working hours, non-work relating coffee break during working hours, work after working hours, and job performance. Our findings are revealed as following: First, non-work-related coffee break has positive effect on performance. Second, non-work-related smartphone usage generally has negative effect on performance, but it has positive effect on performance in personal communication with co-workers. Third, work after working hours has negative effect on performance.