This study aims to identify the crucial prosodic factor for native speakers' naturalness judgement of L2 pronunciation. Prosodic features are known to have more impact on the naturalness of L2 learners' pronunciation than segmental features do. Among prosodic features, timing and pitch are looked at in this study as major prosodic factors which affect native speakers' naturalness judgement of L2 learners' pronunciation. To examine the relative importance of timing and pitch, we synthesized stimuli using STRAIGHT to produce well-controlled timing and pitch errors. Native Japanese listeners assessed the naturalness of these stimuli and the result was compared with the one obtained using natural speech stimuli. The current study obtained the same result as the previous study: that timing is more important than pitch in improving the naturalness of L2 speech.