A new mechanical foam-breaker was developed. It uses the shear force between a fixed orifice plate and a rotating disk (MFUS). Foam-breaking performance of the MFUS fitted to a stirred-tank reactor (STR) was evaluated from changes in foam density and shearing power. The maximum value of the ratio of foam density (after foam breaking) to liquid density was 0.90. Change in foam densities before and after foam breaking in the MFUS was considerably larger than that in existing mechanical foam-breakers with rotating parts at the same level of power for foam breaking. The volumetric liquid-phase oxygen transfer coefficient was measured in the bubble-dispersing and foam-ascending sections of the STR with the MFUS. Comparison of the volumetric liquid-phase oxygen transfer coefficient between the STR with the MFUS and the STR or the foam column in terms of the specific power input demonstrated higher oxygen transfer performance and saving power requirements for the STR with the MFUS.