A biosensor comprising an insulative substrate (1) and an electrode system arranged on the insulative substrate (1). The electrode system comprises thereon a reagent capable of reacting with a test sample. The electrode system comprises functional electrodes (2, 3, and 4) used for testing and assisted diffusion electrodes (12) used for controlling the diffusion of the reagent. The functional electrodes (2, 3, and 4) are arranged between the assisted diffusion electrodes (12). Because the assisted diffusion electrode (12) are capable of overcoming the effect of surface tension of the insulative base board, the reagent is allowed to diffuse smoothly and uniformly on the electrode system and to arrive at a predetermined position.