A time series myoelectric potential signal is acquired, which is representative of time series activity amount of a muscle of a driver when the driver is performing a driving operation on the vehicle, the muscle being involved in the driving operation. Based on the acquired time series myoelectric potential signal, a parameter value indicative of a fluctuation amount of the time-series myoelectric potential signal and an average value of the myoelectric potential signal are obtained. Drivability of the vehicle is evaluated based on the obtained parameter value and the obtained average value.