In this study, neural representation of adult second language (L2) speakers' implicit grammatical knowledge was investigated. Advanced L2 speakers of Japanese living in Japan, as well as L1 Japanese speakers, performed a word-monitoring task (proposed as an implicit knowledge test) in the MRI scanner. Behavioral measures were obtained from aptitude tests for explicit (language analytic ability) and implicit (statistical learning ability) learning. Findings indicate that, although both L1 and L2 speakers recruited neural circuits associated with procedural memory during the word-monitoring task, different brain regions were activated: premotor cortex (L1 speakers) and left caudate (L2 speakers). The premotor cortex activation was weaker in L2 than L1 speakers but was positively correlated with the left caudate activation, suggesting that their grammatical knowledge, while less automatized, was still developing. Behavioral sensitivity to errors was predicted only by explicit language aptitude, which may play a key role in the automatization of grammatical knowledge.