PURPOSE: To detect lowering of awakenness, by setting a reference level from one's brain wave before one drives and comparing it with a brain wave of the driver during driving.

CONSTITUTION: When a contact A of a change-over switch 5 is turned ON and a brain wave amplitude maximum value is held in a hold circuit 6, it is divided by resistors 7, 8, 9 and two level signals 7a, 8a are outputted. When the setting is completed, a contact B of the switch 5 is turned ON and a brain wave signal of a driver who is driving a car is inputted into comparators 10, 11. The comparator 10 compares this inputted signal with the high level signal 7a, while the comparator 11 compares this inputted signal with the low level signal 8a, and when an α wave having a larger amplitude than those of the level signals 7a, 8a is inputted, a comparions signal is outputted. When the comparator 10 outputs a comparison signal or when the comparator 11 outputs a comparison signal equivalen t to a set count number of a counter 12, it is inputted through an OR circuit 13 to a warning circuit 14 to issue a warning.