It is an object of the present invention to provide a biosensor on which a large amount of protein can be immobilized and nonspecific adsorption is less likely to occur with the use of a SAM compound having high water solubility and good performance in terms of supply. The present invention provides a biosensor comprising a substrate on which a hydrophilic polymer having a reactive group capable of binding to a physiologically active substance is immobilized via a compound represented by the following formula (1), said compound at a concentration of 1 mM being dissolved in water at 25.degree. C.: X-L-Y (1) wherein X represents a group capable of binding to metal, L represents a linking group, and Y represents a functional group to which the hydrophilic polymer is bound.