PURPOSE: To attain to reduce an accident in altering a lane, by issuing an alarm by detecting whether an obstruction such as a succeeding vehicle is present or not on a lane intended for alteration.

CONSTITUTION: A control circuit 2 outputs a drive signal to a rear sensor driving circuit 3 when the output of a car speed detecting circuit 1 reaches a set car speed or more to drive a rear sensor 4. The output of the sensor and the outputs of direction indicating switches 5L, 5R in left and right sides are connected to the circuit 2 and, when the obstruction detected by the sensor 4 is present in the side to which the switches 5L, 5R output, an alarm signal is outputted. In this case, the alarm signal of the circuit 2 drives an alarm apparatus 7 through an alarm apparatus driving circuit 6.