Second language (L2) acquisition research has explored the acquisition of various syntactic constraints by L2 learners, one of which is "wanna" contraction. However, there is still a very limited body of research regarding the acquisition of "wanna" contraction, both in first language (L1) and L2. The purpose of the study is to examine adult L2 learners' knowledge of wanna contraction by using a grammaticality judgment task. Ninety-three Japanese learners of English took the grammaticality judgment task which consisted of 24 experimental items and 24 fillers, with a context provided. The experimental stimuli consisted of six conditions: Object extraction (e.g., "Who do you want to/wanna advise?"), subject extraction (e.g., "Who do you want to/*wanna advise Mary?"), and no extraction (e.g., "Do you want to/wanna advise Mary?"), all of which were presented with and without contraction. Subject extraction with contraction was the only ungrammatical condition. The participants were divided into three groups according to their English proficiency as measured by the cloze test: High level (JPN-H), intermediate level (JPN-I), and low level (JPN-L). The same grammaticality judgment task was also given to 41 native speakers of English in order to collect the baseline data. The results showed a proportional increase of learners who were correctly differentiating the contraction patterns and the decrease of those who were overgeneralizing the patterns over the three proficiency groups. This suggests a developmental pattern among the L2 learners with respect to the contraction constraint.