Coarse-to-fine 3D instance segmentation methods show weak performances compared to recent Grouping-based, Kernel-based and Transformer-based methods. We argue that this is due to two limitations: 1) Instance size over-estimation by axis-aligned bounding box(AABB) 2) False negative error accumulation from inaccurate box to the re-finement phase. In this work, we introduce Spherical Mask, a novel coarse-to-fine approach based on spherical repre-sentation, overcoming those two limitations with several benefits. Specifically, our coarse detection estimates each in-stance with a 3D polygon using a center and radial distance predictions, which avoids excessive size estimation of AABB. To cut the error propagation in the existing coarse-to-fine approaches, we virtually migrate points based on the polygon, allowing all foreground points, including false negatives, to be refined. During inference, the proposal and point mi-gration modules run in parallel and are assembled to form binary masks of instances. We also introduce two margin-based losses for the point migration to enforce corrections for the false positives/negatives and cohesion of foreground points, significantly improving the performance. Experimen-tal results from three datasets, such as ScanNetV2, S3DIS, and STPLS3D, show that our proposed method outperforms existing works, demonstrating the effectiveness of the new in-stance representation with spherical coordinates. The code is available at: https://github.com/yunshin/SphericalMask
