An extracellular potential measuring device includes a plate portion having a first surface and a second surface opposite to the first surface, and an electrode provided on the second surface of the plate portion. In the plate portion, a pocket having an opening which opens to the first surface is formed, and a through-hole communicating to the second surface from the pocket. The through-hole communicates from a position which is closer to the opening than a deepest point of the first pocket. The electrode is provided around of the opening of the through-hole. In this device, even if a cell to be examined does not reach the deepest point of the pocket, a cell membrane of the cell can tightly attaches onto the through-hole securely without a clearance. Hence, culture solution inside the through-hole is isolated from culture solution over an upper surface of the plate portion, thereby allowing electrochemical changes caused by activities of the cell to be detected efficiently with a detector electrode.