1. A method of evaluating drivability of a vehicle, comprising the steps of: acquiring a time-series myoelectric potential signal 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; obtaining, based on the acquired time-series myoelectric potential signal, a parameter value representing a fluctuation amount of the time-series myoelectric potential signal; and evaluating, based on the parameter value, the drivability of the vehicle.