The purpose of this study is to test the effect of personal communication activities using smart phone instant messenger during normal office hours on job performance in the workplace. To do this, empirical data were collected by conducting a field survey of smart phone users, and structural equation model was used for the purpose of analyzing the data acquired by the survey. A structural equation model was designed and constructed by such factors like personal instant messaging with the outside friends and co-workers on job performance. In addition, task characteristics set as a moderating effect between personal instant messaging with the outside friends/co-workers and job performance. The results of the analysis are summarized as follows; First, personal instant messaging with the outside friends generally has a negative effect on job performance, in addition in high task interdependence area the former has more effect on the latter. Second, personal instant messaging with co-workers has a positive effect on job performance.