A biosensor comprises, as shown in chart (1), a substrate (1); a conductor layer(2) consisting of precious metal such as gold and palladium or conductor such as carbon; slits (3a, 3b) parallel to the sides of the substrate; slits (4a, 4b) perpendicular to the sides of the substrate; a measuring electrode (5); a counter electrode (6); a detection electrode (7); a spacer (8) covering the measuring electrode (5), the counter electrode (6) and the detection electrode (7) on the substrate (1); a rectangular cut (9) forming a channel through which a sample is supplied; an entrance (9a) to the channel; a reagent layer (12) formed of an enzyme-containing reagent applied to the measuring electrode (5), the counter electrode (6) and the detection electrode (7) which are exposed through the cut (9) in the spacer (8); and a cover (13) over the spacer (8). The biosensor can be provided by a simple technique, and it is of precision and reliability because a uniform layer of reagent, regardless of the composition, covers the electrodes.