PURPOSE: To easily specify an in vivo abnormal site by giving a new error function based on the spatial nature of the background brain wave as the error function utilized in the equivalent current dipole locating method, and obtaining the positions and vector constituents of dipoles with high precision.

CONSTITUTION: About 30 electrode groups 2 are fitted to the head of living body 1, and potentials based on intracranial nerve activities are measured by a potential measuring means 7. The measured potentials are inputted to a computer 6 via an amplifier 3, a multiplexer 4, and an A/D converter 5, current dipoles are assumed at the optional positions in the living body 1 having the nature as a medium by an arithmetic section 6b, the potentials corresponding to multiple electrodes formed by the dipoles are calculated, and the spatial correlation functions of potentials of the electrodes are calculated from the outputs of the potential measuring means 7. Errors between the actual measured values of the potential measuring means 7 and the calculated values of dipole models are calculated based on the calculated values of the correlation functions, and the dipole positions and vector constituents minimizing the error values are obtained and outputted.