This study investigated how argument omissions of a sentence affects language learning by focusing on Japanese object case-markers. To examine this point, we clarify how argument omission of the sentence input affects Japanese children at age four to seven to learn artificial case-markers. Particularly, it explores which of the following two is useful in children: the full-argument sentence with word order information and high processing cost or the argument-omitted sentence with no word order information and low processing cost. Participants watched and imitated four single-action-scenarios while listening to sentences where two artificial case-markers, po and bi, referred to the subject and object, respectively. Half of the participants learned full-argument sentences (“ saru po ushi bi oshita : monkey-NOM cattle-ACC pushed”), and the other half learned argument-omitted sentence with one argument omitted (“ saru po/bi oshita : monkey-NOM/ monkey-ACC pushed”). In a later test, participants completed forced-choice discrimination of scenarios after hearing sentences (OSV, OV, SV and SOV) with either po or bi. A mixed effect model for children’s response (correct answer or incorrect answer) showed that the argument-omitted group of five, six and seven year-old children comprehended OSV and OV sentences well. However, the full-argument group of five, six,seven year-old children comprehended those sentences poorly. On the other hand, the four year olds could not apply it to learning “ bi ”. These results indicate that sentences without case-marker omission was less useful for children in learning object case-marker compared to sentences with case-marker omission. A possible explanation of this result is that children pay more attention to case-markers when they process sentences with argument omission since case-markers are the only reliable cue to distinguish the subject and the object. This study provides the first empirical evidence that argument omission plays a positive role in case-marker learning by the children who learn a language with frequent argument omission.