PURPOSE: To culture microorganisms for decomposing bistributyl tin oxide (TBTO) in a large amt. by adding glucose, butyl-contg. org. matter and TBTO successively into the sea water, then culturing the microorganisms under aerobic conditions.

CONSTITUTION: The sea water, sea bottom soil, etc., contg. the TBTO are introduced into a bioreaction vessel V1 and after the glucose is added thereto, air is fed therein to culture and propagate the various microorganisms existing in the sea water, etc. The culture liquid circulates in an arrow direction and the microorganisms therein begin to stick to carriers packed in V1-2. The microorganisms having the ability to cut and decompose butyl groups begin to propagate and stick when butyl group-contg. org. matter such as butanol is added to the liquid and the culture is executed under the aerobic conditions. The microorganisms which can decompose the TBTO can be propagated and stuck if the TBTO is further added thereto.