The cell observation using a conventional well plate takes much costs. Each well 3 being opened at a top plate 2a of the well plate 1 has a diameter reduced portion 32 which inner circumferential surface 32a is conically hollowed. At a lower portion of the diameter reduced portion 32 is formed an inserting hole 33. In a detecting portion 4, an outer circumferential surface of a round-bar reference electrode 41 is covered by an insulating portion 42. The detecting portion 4 has an upper end portion 4a and a lower end portion 4b which outer circumferential surface is exposed to an outside without being covered by an insulating portion 42. On an upper side above the lower end portion 4b is formed a diameter expanded portion 4c in which an outer circumferential surface of the insulating portion 42 is covered with a measuring electrode 43. The detecting portion 4 is fixed to a well 3 whose diameter expanded portion 4c is inserted into an inserting hole 33 and its upper end portion 4a is contained in the well 3. In the detecting portion 4, the upper end surface 43a of the measuring electrode 43 is communicated with an inner circumferential surface 32a of the diameter reduced portion 32 positioned in an edge surrounding portion of the inserting hole 33.