This study investigates the emergence and development of the discourse-pragmatic functions of the Japanese subject markers "wa" and "ga" from a usage-based perspective (Tomasello, 2000). The use of each marker in longitudinal speech data for four Japanese children from 1;0 to 3;1 and their parents available in the CHILDES database (MacWhinney, 2000) was coded and analyzed. Findings showed that the four children initially used "wa" as a "wh"-question marker. They then gradually shifted its use to convey the proposition of given information. In contrast, the use of "ga" varied among the children. One child used "ga" with dynamic verbs in the past tense to report events he witnessed/experienced, while the other three children used it with a particular stative predicate in the present tense, expressing their subjective feeling toward referents. Findings were explained by the frequency of input to which the children had been exposed.