Self-regulated learning explores learners' complex intrinsic factors, including their learning strategies and motivation to learn. It offers learners the opportunity to become autonomous--a situation that is essential for ensuring that Japanese students acquire English proficiency. This study aims to explore the factors involved in self-regulated language learning of Japanese senior high school students and to identify the different types of self-regulated English learners. A 45-item questionnaire covering learners' cognitive, behavioral, and affective aspects was distributed among 1076 students from seven prefectural senior high schools in Japan. Exploratory factor analysis confirmed the presence of five factors. Cluster analysis was used to classify students into four distinct profiles, and a follow-up interview was conducted with six students extracted from each cluster group. The findings confirmed that Japanese high school students have very complex internal factors that affect their learning. These results offer pedagogical implications and emphasize the importance of EFL teachers' roles in Japan. They suggest that teachers should strive to understand each student's readiness and internal factors in order to transform them into autonomous learners.