1. A ranking device for ranking a plurality of research objects based on differences in a degree of a rising attention of a subject to each of the plurality of research objects, configured to: classify target and non-target for brain wave data of event related electrical potentials, wherein the brain wave data is measured by a brain wave monitor with electrodes attached on a head of the subject; calculate a weighting factor of discriminant equation based on data point of time series data and the number of a channel according to a number of measured positions where the brain wave data of the even related electrical potential is acquired when each of the plurality of research objects is the target; calculate a discriminant point for each product as the target based on the discriminant equation; and rank the research objects based on the discriminant point calculated for each product.