Media Richness Theory(MRT) and Social Influence Theory(SIT) are regarded as a representative theory of media use. MRT hypothesizes that its media characteristics is immovable and SIT, on the other hand, is hypothesized by social influence. The purpose of this study is to enhance the power of explanation on the media use of organization through the combination of MRT and SIT. To fulfill this purpose, I set the following hypotheses: (1) The more experience and skill of electronic mail they have, the bigger perceived media richness it is. (2) The bigger social influence of electronic mail they have, the bigger perceived media richness it is. (3) In higher equivocal task, the bigger perceived media richness they have, the higher ratio of electronic mail use it becomes. (4) The more experience and skill of electronic mail they have, the higher ratio of electronic mail use it becomes.