To provide an electrode and a bioreactor constituted so as to enable multistage oxidation-reduction reaction between coenzyme and the electrode, to provide a mediator between the coenzyme and the electrode in order to provide a proper electron transmission field not obstructing the activity of the coenzyme, to realize the recycling of the coenzyme in order to enable continuous measurement, that is, to regenerate oxidation type coenzyme to reduction type one to dispense with the removal of oxidation type coenzyme and, in its turn, to measure the activity of enzyme easily and simply with high sensitivity, and also to provide a measuring method of enzyme activity using them.

In the electrode for measuring the activity of coenzyme depending type enzyme, an enzyme layer 13 is laminated on the surface of a conductor 11 through the mediator 12 capable of receiving and giving electrons so that the coenzyme 14 is brought into contact with the enzyme layer 13 and the enzyme layer 13 acts on the giving and receiving of electrons of the mediator 12 and the oxidation-reduction reaction of the coenzyme 14.