A 63-year-old man presenting remittent fever and multiple arthralgia was diagnosed as adult-onset Still's disease (AOSD), and started with prednisolone treatment. However, he suddenly developed loss of consciousness, paresis of the right upper extremity and aphasia shortly after the treatment. We detected an increased signal of brain tissue lactate at the branch territory of left middle cerebral artery by MR spectroscopy (MRS), but no lesions by diffusion-weighted nor T2-weighted MRI, suggesting acute brain ischemia of embolic mechanism. Most of the symptoms resolved in a couple of hours after the onset, showing spectacular shrinking deficit (SSD). The patient also revealed complication of antiphospholipid antibody syndrome (APS), which may be associated with ischemic event. This is the first case of acute brain ischemia with SSD, which occurred in AOSD with APS. MRS was superior to diffusion MRI in detection of acute brain ischemia.