PURPOSE: To obtain a fine platinum electrode having solidity and durability by coating a zirconium oxide substrate with a platinum membrane.

CONSTITUTION: A platinum membrane with thickness of about 0.1μm is applied to a zirconium oxide substrate 12 to form comb-shaped platinum electrodes 14a, 14b, Since zirconium oxide and platinum have a good affinity, even when voltage is applied for the sake of measurement or even after long-time use, the platinum membrane is not released from the substrate 12. The electrodes 14a, 14b are respectively connected to terminals 16A, 16B by lead wires using indium solder 18 and the terminals 16A, 16B are covered with epoxy resin coating 20 and a part of the electrodes 14a, 14b and the coating 20 are further covered with polymer coating 19. Therefore, the fixing of the terminal parts and the prevention of the short circuit between the lead wires through indium solder become possible. When the interval between the electrodes 14a, 14b is set to about 0.1mm and voltage is applied across the terminals 16A, 16B, the resistance value between the terminals lowers and much concn. data is obtained from a sample solution.