Driver condition detecting device, in-vehicle alarm system and drive assistance system

When there is a preceding vehicle in a traveling direction of a subject vehicle, an ECU (100) computes a driver condition coefficient (K) that represents degree of change in the size of an image of the preceding vehicle as viewed from the driver is computed. The driving condition of the driver is detected from this driver condition coefficient (K) and used to provide an alarm indicative of the driving condition and to control vehicle travel condition.