A 54-year-old male previously diagnosed as having a lung squamous cell carcinoma T2N2M0 in stage III A was readmitted to our hospital, as he was complaining of a gait disturbance. A brain CT, an MRCT scan, and vertebral arteriography revealed that the lung carcinoma had metastasized as a solitary cystic formation in the right cerebellum. Thus, a craniotomy, a cyst fluid aspiration, and tumoral resection was performed. Histopathological findings of the cyst wall indicated a squamous cell carcinoma, and the protogram of the cyst fluid found it similar to that of the serum. These findings suggest that destruction of the blood-brain-barrier by the invasion of the carcinoma caused an exudation of serum, resulting in the accumulation of cyst fluid.