An arousal level judging apparatus has a skin potential level (SPL) detection device, a potential change detection device for detecting occurrance and convergence of a pulse of the signal from the SPL detection device, the pulse being caused by skin potential response (SPR), a storing device for storing the SPL value as a reference value, and a comparator for comparing SPL with the reference value to detect a SPL decrease to detect an arousal level decrease of a driver, etc. The reference value is renewed at every occurrance of SPR. When the SPL value indicates that the arousal level of the driver has been decreased, an alarm is caused to wake the driver up.