A simple test was designed to detect the disorder of vigilance, and its efficiency in clinical use was examined. The subject was instructed to tap a table continuously at the rate of once per second with a pencil held in his normal hand. The examiner, relying on a stopwatch, counted the number of tapping sounds per block of ten seconds for a duration of five minutes. As the index of this task, the mean and SD values of tapping sound numbers for thirty blocks (i. e., five minutes) were used.     It was expected that performance of the task would fluctuate if vigilance was disturbed. For clinical examination, this task was applied to patients having left unilateral spatial neglect (left USN), who tend toward distraction and poor concentration in their daily life.     In contrast to the control group, many patients in the left USN group showed a high mean and a large SD value.     These results show that this task reflected the behavioral features of the left USN patient stated above. They also suggest that left USN patients cannot adequately control their own behavior continuously (i. e., sustained attention or vigilance disorder).