The aim of the study is to investigate the causee case marking by Japanese L1 learners of Korean. Twenty Korean native speakers (NK) and 54 Japanese L1 learners participated (24 high level learners, HL and 30 low level learners, LL). The task was acceptability judgment task composed of 48 causative sentences. Linguistic contexts of sentences were divided by verb transitivity (transitive causation and intransitive) and type of causation (syntactic and morphological). The types of case marking were nominative, accusative, and dative. The result showed proficiency variability among LL and HL. LL showed different acceptability judgment from NK by taking permissive attitude in ungrammatical nominative and dative case marking contexts. Their L2 responses were not fully reflecting their L1 norm nor exhibiting target language-like norm. This indicates that L1 is not the fundamental resources of lower level L2ers when they face poverty of stimuli. They continue to test interlanguage hypothesis by keeping themselves open to L1 and L2 phenomenon. HL's acceptability judgment was not significantly different from both of NK and LL which indicated that they were in the middle of acquiring Korean causee marking even when they had arrived at the advanced level of Korean proficiency. This implies that explicit instruction on causee marking needs to be provided in L2 Korean classroom. (Ewha Womans University)