An enzyme electrode unit having, on the surfaces of the base electrodes thereof, an enzyme-immobilized membrane for oxidizing or reducing a target substance to be measured, and a diffusion-limiting membrane unit of a two-layer structure disposed on the surface of the enzyme-immobilized membrane, only the diffusion-limiting membrane having a lower target substance diffusion limiting effect being replaceable, thus maintaining substantially constant the general target substance diffusion limiting effect of the diffusion-limiting membrane unit, even after the replaceable diffusion-limiting membrane has been replaced.