Research into the effects of extensive reading (ER) has surged over the past few decades. However, many of these studies are limited by their lack of control over how the ER treatment is conducted. Furthermore, experimental and quantitative studies that investigate the possible effects of ER on the reading skills of learners of Japanese have yet to be fully explored. The goal of this study was to investigate the possible effects of ER on the reading rate development and comprehension abilities of learners of Japanese as a foreign language (JFL). Using a quantitative single-case study method, eight intermediate learners of Japanese were monitored while they engaged in ER and strictly adhered to ER principles over 2.5 to 4 months. Results showed that participants’ reading rates increased significantly following the monitored ER treatment and that comprehension abilities were not hampered by an increase in reading rate. Pedagogical implications will be discussed.