Disclosed is a biological state determination device which precisely determines the biological state of the driver of a vehicle. The biological state determination device estimates a driver biological state, which is the biological state of the driver, in accordance with information indicating the biological state of the driver, and a weighting is set for each driver biological state in accordance with information indicating the possibility of incorrectly estimating the driver biological state. Since each driver biological state has a weighting added thereto by means of the set weighting, the weighted driver biological state is information in which the possibility of incorrect estimation has been taken into account. It is therefore possible to improve the accuracy with which the biological state of a vehicle driver is determined since whether or not there is an abnormality in the biological state of the driver is determined in accordance with a plurality of weighted driver biological states.