By the use of grammaticality judgment tasks in two conditions (untimed and rapid), the present study examined Japanese EFL learners’ explicit and implicit knowledge about the countability of common and material nouns. It is well known that Japanese EFL learners exhibit long-lasting difficulty in attaining knowledge of countability. However, previous studies have not taken the explicit/implicit distinction of L2 knowledge into account. Hence, the present research conducted a grammaticality judgment study, targeting highly proficient Japanese EFL learners (N = 18). The stimuli consisted of both grammatical and ungrammatical sentences including number agreement errors with two conditions (common vs. material nouns; k = 12 for each). The results of a two-way ANOVA (task type: untimed and rapid, by the stimuli type: common and material nouns) clearly revealed that there was a statistically significant interaction between the two factors, showing a large effect size. This suggests that (a) there was no effect of task type on the material nouns, which might lead to the conclusion that the participants were not aware of the countability of material nouns, even as explicit knowledge; (b) the accuracy score of common nouns substantially declined, which means that the countability of common nouns was not fully acquired as implicit knowledge.