To detect danger such that a driver is calling through a cellular phone in a vehicle so as to warn of the danger.

A sight direction detector 5 which is provided to a hand-free telephone system incorporating a microphone 1 with a switch, a speaker 2, a hand-free control unit 3 and a cellular telephone 4, is composed of a CCD cameral part 6 and an image processing part 7, and an infrared LED 6a coaxial with a lens, and an infrared LED 6b non-coaxial with the lens, are alternately turned on and on. Thus obtained image data as to the eyes of the driver is transmitted to the image processing part 7. The image processing part 7 processes the data so detect a direction of the sight of the driver, and accordingly, warning is issued from the speaker 2 when it is determined that the drive gazes only in a one direction for a long time, or faces in a direction other than a driving direction for a long time.