This study aims to examine the effect of relational mobility on the Willingness to Communicate (WTC) in English of Japanese people. Previous studies have focused on internal factors such as perceived competence and personality to predict the WTC, but did not reveal which environmental factors influenced these factors. This study focused on relational mobility as a socioecological factor. A pilot survey showed that to predict the WTC in English of Japanese people, perceived communication competence in English had the strongest positive effect, as in previous studies. Study 1 showed that relational mobility positively influenced the WTC via perceived competence, targeting university students. Study 2 showed that, targeting different university students, the mediation effect found in Study 1 was confirmed. In addition, we examined whether relational mobility enhanced the WTC via a decreased evaluation concern and increased the perceived competence or not, but the whole indirect effect was not confirmed.