A 75-year-old man was referred to our hospital with a thrombosed giant middle cerebral artery aneurysm manifesting as progressive memory disturbance and disorientation. Magnetic resonance imaging and conventional angiography revealed a partially thrombosed giant aneurysm of the left middle cerebral artery bifurcation and edema of the adjacent brain which had enlarged compared to 3 months before. Surgery was performed through a left frontotemporal craniotomy. After exposure of the aneurysm neck, we tried to apply a clip, which slipped due to the intraaneurysmal thrombus. Intraoperative motor evoked potential monitoring showed decreased amplitude. Therefore, the aneurysm dome was incised and the intraaneurysmal thrombus near the neck was shaved with the ultrasonic aspirator, followed by neck clipping of the aneurysm. The residual thrombus was safely removed. Transient right hemiparesis was observed after surgery, but his memory disturbance gradually improved. Giant thrombosed aneurysm can be treated by reduction of the thrombus from the far side to the lumen to reduce the duration of parent artery occlusion required for clipping.