A drowsiness alarm system for a vehicle detects the driver's increasing state of drowsiness on a basis of abnormal steering change repetitions for a fixed period of time in a monotonous driving state defined, for example, as the gear shift lever being placed at an overdrive or top position. The alarm system produces an alarm to the driver to prevent the driver from increasing the state of drowsiness. The drowsiness alarm system according to the present invention comprises: (a) a steering signal generator which generates a signal corresponding to the change of an angle through which the steering wheel is rotated; (b) a monotonous driving-state detector which detects and signals that the vehicle is traveling in a monotonous driving state; (c) a discriminator which compares the change of steering signal level from said steering signal generator for a fixed period of time with a reference value; and (d) an alarm pulse generator which outputs an alarm pulse into an alarm circuit producing an alarm in response to the input alarm pulse when the output signal from the discriminator indicating the change of steering signal for the fixed period of time exceed the reference voltage.