PURPOSE: To enable the recovery and reutilization of a biocatalyst such as microbial cell, enzyme, etc., easily, by immobilizing the catalyst together with a magnetic substance using a bonding means, contacting the immobilized catalyst with a substrate to induce the reaction and recovering from the reaction mixture with magnetic force.

CONSTITUTION: For example, methane-fermentation microorganism is included and immobilized with agarose gel containing powdery magnetite. The immobilized catalyst is put into a substrate to effect the methane fermentation, and after the reaction, recovered with a rotary member 31 having electromagnets arranged to the outer circumference.