In second or foreign language (L2) extensive reading (ER) studies, learners have been encouraged to read easy books despite the lack of consensus concerning how to define book difficulty. In light of previous studies reporting that book difficulty can play an important role in learners' affect in ER (e.g., Bahmani & Farvardin, 2017; Chiang, 2016; Yang et al., 2021), the present study explored the relationship between perceived book difficulty and pleasure experiences from the perspective of the flow theory (Csikszentmihalyi, 1975/2000), as an important reading purpose in ER is for pleasure. Ninety-nine Japanese university students participated in a flow questionnaire survey, five of whom also participated in the follow-up interviews. It was found that some characteristics of flow were found in relation to the difference in perceived book difficulty, and that the participants' perceptions were affected by some factors related to the flow theory.