Native languages (L1s) are tacitly assumed to be complete and stable in adults. Here we report an unexpected individual variation in judgment of L1 regarding Japanese sentences including honorification, and further clarify its neural basis with functional magnetic resonance imaging (fMRI). By contrasting an honorification judgment task with a spelling judgment task, the lower performance group showed more extensive activation in the left inferior frontal gyrus than did the higher performance group. Moreover, activation in the left dorsal and ventral triangular parts negatively correlated with the performance of the honorification judgment task. This modulation pattern demonstrates that cortical activations recruited for sentence processing depend on individual performances even in L1. (Contains 4 tables and 4 figures.)