A microbial BOD sensor using immobilized Hansenula anomala was prepared for the estimation of BOD. The sensor voltage was increased with time and increasing concentration of GGA when it was inserted in a sample solution. A linear relationship was obtained with a correlation coefficient, 0.998 between the concentration of standard GGA solutions and dV/dt by using the initial change of voltage, in which the response time was 20 min. It could be concluded that the oxidation of GGA conformed to a first-order kinetics. Therefore, the good linearities were also observed at various times. This sensor showed the best linearity at 30 min.