Recent experimental results regarding the action potential duration restitution curve have explained the transition from ventricular tachycardia (VT) to fibrillation (VF) in terms of spiral wave (SW) meandering and breakup. However, it remains unclear whether VF always has a steep restitution curve. The present study was designed to test the hypothesis that afterdepolarizations occur at excitable gaps during VF and affect the SW dynamics, even if the restitution curve is gentle. Homogeneous and isotropic 3-dimensional tissue was simulated with a LRd model. Because of the gentle restitution curve, it was not expected that SW instabilities would occur in this condition. In the tissue, a stationary SW reentry was initially observed; however, afterdepolarizations erupted from the excitable gap near the SW tip, and the SW then meandered widely. Following that, afterdepolarizations erupted far from the SW tip, resulting in SW breakup. In this manner, the wave dynamics degenerated into a chaotic state within a few seconds. Furthermore, not only triggered activity but also subthreshold afterdepolarizations were found to cause SW instabilities. These results suggest that afterdepolarizations may play an important role in the transition to VF and that the mechanism is independent of restitution properties.