A device for preventing a vehicle from being started includes a breath gas analyzer (5), which is arranged within the vehicle and is designed to detect the breath alcohol content of an operator. A repeated test can be performed, without compromising the operator, with a control unit (1), which is connected to the breath gas analyzer (5), a motion sensor (4) and an ignition system (3) of the vehicle, to request a breath alcohol test at presettable times tW in such a way that the breath alcohol test is performed with the vehicle stopped. The control unit (1) is also designed to preset, beginning from the time tW, a time interval as a latency period tL within which the vehicle must be stopped.