It is possible to determine the biological state of the driver of a vehicle with high accuracy. In a biological state determination device, since a driver biological state which is the biological state of the driver is estimated on the basis of information representing the biological state of the driver, a weight is set for each driver biological state on the basis of information representing the level of possibility of erroneously estimating the driver biological state, and each driver biological state is weighted on the basis of the set weight, the weighted driver biological state becomes information in which the possibility of erroneous estimation is taken into consideration. The presence/absence of an abnormality in the biological state of the driver is determined on the basis of a plurality of weighted driver biological states, thereby improving determination accuracy of the biological state of the driver of the vehicle.