A 22-year-old woman was admitted to our department after developing a headache. The neurological findings were unremarkable on her first visit, but CT demonstrated a lot of low-density masses in the subarachnoid space. The largest mass was in the right sylvian fissure. These lesions appeared hyper-intense in T1, T2 and diffusion-weighted MR images. A right frontotemporal craniotomy was performed to remove the main mass lesion in the right sylvian fissure. During surgery, thickening of the arachnoid membrane and floating oily globules were seen in the subarachnoid space. The histopathological examination revealed that the tumor was a dermoid cyst. Follow-up MRI revealed that some of the small lesions had moved since the operation. These findings suggested that the tumor was a ruptured dermoid cyst. The patient's postoperative course was uneventful and her headache disappeared completely.