In this paper, the apparent viscosity which is one of the most important apparent mean physical properties in a two-phase flow, has been measured by a specially designed rotation viscometer. The experiment has been carried out using a small air bubbles-liquid tow-phase mixture. From the experimental results, the relationship between the specific viscosity ηsp and the void fraction φ has been expressed with an experimental formula involving the dimensionless number Ta, which shows the amount of the deformation of bubbles in shear flow.