This study examined foam breaking characteristics of a mechanical foam-breaker using shear force (MFUS) between a fixed orifice plate and a rotating disk fitted to a stirred-tank reactor (STR). The effects of operating conditions on the liquid holdup in foam phi(L) and the volumetric rate of foam flow Q(a), which reflect the foaming behavior of the STR, were revealed. The foam-breaking behavior of the MFUS was investigated from changes in the foam density after foam breaking (p(f))(c), which remained constant at a disk rotational speed N, transitional disk rotational speed N-t, and shearing power P-s. The value of (p(r))(c) decreased concomitant with the increase in the mass flow rate of liquid in the foam flowing into the orifice, w(L). The N-t became larger concomitant with the lower average ascending velocity of the foam U-a and higher phi(L). The magnitude of P-s was related to that of w(L). Empirical equations for predicting (p(f))(c), N-t and P-s were obtained based on those results.