This study examined number marking comprehension among Japanese learners of second language (L2) English, whose first language (L1) does not have an obligatory number marking system. The study conducted an online sentence comprehension experiment with 96 L1-Japanese learners and 32 native speakers of English, wherein participants engaged in a self-paced reading with Stroop-like number judgment tasks. Participants were required to determine the number of single words in stimuli (e.g. "cat"/"cats," one word; "the cats"/"the cat," two-word sets), and their judgment time was measured for singular and plural words. The results indicated that both groups took more time to judge single plural nouns, suggesting that Japanese L2 learners of English automatically activate plurality in online sentence comprehension as native speakers do. In contrast, neither group showed an interference effect of singularity in judging singular two-word noun sets ("the cat"), unless the singularity is explicitly marked by indefinite article ("a cat"). The lack of interference may be because of unmarkedness of singularity.