To provide a method for inhibiting sludge formation that is able to efficiently and safely inhibit formation of petroleum sludge, such as crude oil, and to obtain a safe and inexpensive inhibitor of petroleum sludge formation.

This petroleum sludge inhibition comprises adding to petroleum microbial cell body, cultured matter or a preparation thereof of a microbial strain preferably belonging to Acinetobacter, Acromobacter Bacillus, Brevibacterium, Corynebacterium, Mycobacterium, Nocardia, Pseudomonas, Rhodococcus, Serratia, Candida, Torulaspora, Yarrowia, Talaromyces, Escherichia and Saccharomyces, thereby inhibiting formation of petroleum sludge. Furthermore, these bacteria, or their cultured matters or preparations are compounded with other petroleum sludge inhibitors.