Provided is a technique by which a biosensor that is able to be stored for a long period of time can be easily manufactured by having an enzyme layer and a mediator layer contain a hydrophilic polymer having no double bond of oxygen atoms. Since an enzyme layer (106b) and a mediator layer (106c) are formed together as a reaction layer (106), it is not necessary to individually form the enzyme layer (106b) and the mediator layer (106c) on a cover layer (130) and on an electrode layer (110) respectively as in the case of conventional biosensors, and thus the reaction layer (106) can be formed easily. Since the enzyme layer (106b) and the mediator layer (106c) respectively contain a hydrophilic polymer having no double bond of oxygen atoms, reaction between the enzyme and the mediator is suppressed and reduction of the mediator in the mediator layer (106c) is also suppressed during storage. Consequently, a biosensor (100) that is able to be stored for a long period of time can be easily manufactured.