Cavernous internal carotid artery (ICA) aneurysm complicated by simultaneous and spontaneous formation of thromboses in the aneurysm and the parent artery is a rare clinical condition. Although the majority of patients have good outcomes, some patients experience severe ischemic stroke. Here, we report a case of symptomatic large cavernous ICA aneurysm complicated by rapid growth of an intra-aneurysmal thrombosis with simultaneous parent artery thrombosis. A 68-year-old female presented with sudden-onset diplopia, right ptosis, right conjunctival hyperemia, and paresthesia of the right face. Magnetic resonance imaging (MRI) and digital subtract angiography (DSA) revealed the presence of a large partially thrombosed aneurysm in the cavernous portion of the right ICA. We planned endovascular embolization using a flow-diverting (FD) stent. Dual-antiplatelet therapy (DAPT) with aspirin and clopidogrel was started 2 weeks prior to treatment. Although the neurological state was stable, DSA conducted on the day of the endovascular treatment showed rapid growth of an intra-aneurysmal thrombosis and de novo thrombosis in the parent artery. Direct aspiration was performed via a distal support catheter with proximal blood flow arrest using a balloon-guide catheter, and the FD stent was successfully deployed. The patient's symptoms improved postoperatively and DSA obtained 12 months after the procedure confirmed complete occlusion of the aneurysm. Although the exact mechanism of simultaneous thrombosis formation of the aneurysm and its parent artery remains unclear, it is important to recognize that rapid growth of the thrombosis increases the risk of ischemic stroke.