Behavior of myoblast has been investigated under the uniform shear flow in vitro. The culture medium was sandwiched with the constant gap between the lower stationary culture plate and the upper rotating parallel plate to make a Couette type of the shear flow. By the rotating speed of the upper disk, the wall shear stress (τ) on the lower culture plate was controlled. C2C12 (mouse myoblast cell) was used in the test. After cultivation without flow for 24 hours for adhesion of cells on the lower plate, τ < 2 Pa was continuously applied on cells for 7 days in the incubator. Behavior of each cell was traced at the time lapse images observed by an inverted phase contrast microscope placed in an incubator. Experimental results show that cells differentiate to myotubes under τ < 2 Pa. Both the cell cycle and the cell length tend to scatter in the wider range, and the longitudinal axis of each cell tends to align to the flow direction by the shear stress of 1 Pa. The experimental system is useful to study quantitative relationships between the shear stress and the cell behavior: deformation, orientation, and differentiation.