This study employed the mixed method explanatory sequential design to investigate Japanese fourth grade students' reading motivation. A total of 102 students first completed the Motivation for Reading Questionnaire (MRQ) to investigate the differences in reading motivation among different dimensions in three categories (Competence and Self-Efficacy, Goals for Reading, and Social Reasons for Reading). Then, all students were categorized into three groups: high, medium, and low. Four students from each motivation group were selected for semi-structured interviews to specify critical factors impacting their reading motivation. Both quantitative and qualitative results clearly indicate the cultural influence on students' motivation to read, including collectivist culture and educators' and parents' reading beliefs. This research supports the view of sociocultural theories that an individual's learning and development are closely tied to the social and cultural contexts in which the learner is situated.