The present invention relates to a BOD analyzer comprising a microbe sensor containing an oxygen electrode and a microbe membrane. The microbe membrane is made by immobilizing microorganisms belonging to the genus Klebsiella in membrane. Specifically, the present invention relates to a BOD analyzer comprising flow cell equipped with a microbe sensor containing an oxygen electrode and a microbe membrane; and a liquid passage which is connected with the entrance of the flow cell and which is equipped with an outlet. The microbe membrane is made by immobilizing microorganisms belonging to Klebsiella oxytoca 12092 strain in a porous hydrophilic membrane having an average pore size of 0.65-3 .mu.m in diameter by using at least one gelating agent selected from the group consisting of alginic acid or salts thereof, agar, gellan gum, xanthane gum, gelatine, carageenan, locust bean gum, methylcellulose, pectin, and pullulan. The present invention also relates to improved methods of analyzing BOD by using the BOD analyzer.