A driver state assessment device is provided in which when driver model identification means (M2) identifies a driver model showing a driver's input/output relationship using a difference between a target azimuth and an actual azimuth as a driver's input and an actual steering angle as a driver's output, driver model amount of operation acquisition means (M3) acquires a driver model steering angle by inputting a current azimuth deviation into the driver model, and driver state assessment means (M4) calculates a difference between a current actual steering angle and a driver model steering angle as a residual error and assesses the driver's state based on the residual error. Therefore, since the residual error is an index that represents a fluctuation component, a noise component, a non-linear component, etc. obtained from the driver model, it is possible to assess, with high precision, the driver's state, in particular a low wakefulness state of the driver, based on this residual error.