A BOD analyzer is prepared having 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 the membrane. Specifically, the BOD analyzer has a flow cell equipped with a microbe sensor containing an oxygen electrode and a microbe membrane, and a liquid passage which is connected with an 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 alginic acid or salts thereof, agar, gellan gum, xathane gum, gelatine, carageenan, locust bean gum, methylcellulose, pectin, or pullulan. The BOD analyzer can be used for batch or continuous BOD analysis and enables carrying out BOD analysis in a short period of time.