The present invention includes (i) a right instantaneous phase determining section (12R) for successively determining an instantaneous phase ÆR(f, t) of a right electroencephalography signal XR, (ii) a left instantaneous phase determining section (12L) for determining an instantaneous phase ÆL(f, t) of a left electroencephalography signal XL, (iii) a phase synchronization index calculating section (13) for successively calculating a phase synchronization index PSI(f, t), and (iv) a video signal generating section (14) for generating a video signal representing a video image whose content changes in correspondence with the phase synchronization index PSI(f, t).