The current research targeted Japanese high school EFL students (N = 296) to examine the causal relationships among vocabulary learning strategies (VLSs), self-efficacy, self-regulating capacity in vocabulary learning (SRCvoc), and vocabulary size gains. To this end, we used structural equation modeling (SEM) and formulated three hypotheses for the structural model: (1) SRCvoc promotes VLS use and subsequent vocabulary size gains; (2) self-efficacy fostered by SRCvoc reinforces further VLS use; and (3) success in vocabulary size acquisition is associated with self-efficacy increase. The results of the SEM support the structural model and its three hypotheses for the students in this study. However, the weak relationships found among the four variables were in contrast to those of prior studies targeting students in contexts other than Japanese high schools. By comparing the results of previous and current research, we consider the possibility of influences of different variables on vocabulary learning in different learning environments.