Method, system and device for assisting diagnosis of autism which enable assistance of early detection and early definite diagnosis of autism (especially in infants) based on objective evaluation, using a conventionally suggested “eye-gaze detection technique”. The method for assisting diagnosis of autism of a subject uses an eye-gaze detecting unit, the unit having at least an imaging camera portion. The method includes displaying, at the position in front of the subject along the direction of the eye gaze of the subject, an illusionary image contained in a plane of an illusion-causing image on a displaying device. The eye-gaze position information of the subject looking at the plane is detected, and the eye-gaze position information of the subject is input into an eye-gaze position information storage portion The eye-gaze positions of the subject can be evaluated using a algorithm for assisting diagnosis of autism.