The author analyzed the reading histories of 42 students in an ER program in Japan. They joined the program at 15 years old and read a million words or more of English texts for five to seven years. They were divided into three groups depending on their TOEIC score increase rate: 57, 121, and 187 points per million words on average. Their readability levels (Yomiyasusa-level: YL) and the total words read were analyzed. The lowest score-increase group had read 2.2 million total words and was reading the texts of YL 3.2 on average. In comparison, the highest score-increase group had read ‘fewer’ 1.2 total words and was reading ‘easier-to-read’ text of YL 2.1 when they both scored TOEIC 500. The result suggests that the optimum readability levels for Japanese elementary-level EFL learners are around YL 2.5 or even lower when their TOEIC scores are 500.