We propose a method that is effective in tracking 3D hand poses occluded by a real object. Since existing model-based tracking methods rely only on observed images to estimate hand joints, tracking generally fails when the hand joints are largely invisible. This problem becomes more prevalent when the tracked hand is grabbing an object, as occlusion by the object makes it harder to find a proper correspondence between the hand model and observation. The proposed method utilizes the occluded part of the hand as additional information for model-based tracking. The occluded depth information is reconstructed according to the geometric of the object and model-based tracking is employed based on particle swarm optimization(PSO). We demonstrate that the reconstructed depth information improves the performance of tracking an object-grabbing hand.