disclosed is a high performance biosensor free from the effect of any components existing in a sample solution other than a substrate . the biosensor comprises an electrically insulating base plate , an electrode system including a working electrode and a counter electrode provided on the base plate , and a reaction layer containing a hydrophilic polymer , an enzyme and an electron acceptor located over the electrode system . the reaction layer contains an excess amount of hydrophilic polymer in a range of 150 to 1 , 000 % by weight of the enzyme .