A novel 2,4‐dichlorophenoxyacetate (2,4‐D) biosensor system was constructed with a reactor for microbial degradation and a flow cell based on a tyrosinase‐modified graphite electrode for product detection. The microorganism, isolated from the agricultural soil collected at northern Kyusyu Island and identified as Ralstonia sp. was employed as the 2,4‐D degrader. Immobilization was performed with a glass column packed with silica gel particles by circulating Luria‐Bertani medium containing 2,4‐D inoculated with the bacteria. The degradation capability of the immobilized cells packed in the reactor was confirmed by circulating a mineral salt medium containing 2,4‐D and monitoring the decrease in 2,4‐D content. The tyrosinase electrode was employed to monitor phenolic and catecholic compounds, since it could be presumed that 2,4‐dichlorophenol and 3,5‐dichlorocatechol could be produced as intermediates in the degradation of 2,4‐D by Ralstonia sp. The flow cell of three electrodes configuration was assembled by using the enzyme electrode as a working electrode. Consequently, amperometric response current could be observed by injecting 2,4‐D solution with phosphate buffer as the mobile phase at the applied potential of 0.5 V vs. Ag/AgCl. The sensitivity of the system was shown to depend on the composition of the mobile phase by comparing the sensitivities obtained with phosphate buffer and mineral salt medium as the mobile phase.