A warning signal controlling device for blind zones when vehicles making a lane change is provided. The device is disposed inside each inner side of each rearview mirror of the vehicle, and comprises a lens masked by PMMA which is black and opaque, a mirror utilizing a light guide made of PMMA which is transparent, and a base assembly consisting of LED lamps, a PCB board and a base. The lens and the mirror are obtained by two-color molding, the PCB board is secured to the base, LED lamp holders are provided on the PCB board, the LED lamps are secured in the corresponding LED lamp holders and the base assembly is secured to the lens by ultrasonic welding.