An information processor includes a brain wave sensor adapted to output a brain wave signal by measuring brain waves of a subject, an external sensor adapted to sense the surrounding environment of the subject, an environmental change determination section adapted to determine, based on a sensor signal output from the external sensor, whether a stimulus resulting from a change in the surrounding environment of the subject beyond a steady state has been applied to the subject, an event-related potential detection section adapted to detect an event-related potential for the stimulus in the brain wave signal if it is determined that the stimulus has been applied to the subject, and a calculation section adapted to calculate, based on a feature quantity obtained from the detected event-related potential, a value representing the focus level of the subject on a target.
