When the effects of viscosity is dominant in the peristaltic pumping, the normalized pressure rise at zero time-mean flow is independent of Reynolds number. The pressure rise increases nonlinearly with Reynolds number at high value of it caused by the inertial effects. The experimental results indicates that the critical range of Reynolds number separating both tendency is about 1020.In this report, the flow patterns in the region are experimentally analyzed by means of flow visualization. The method is the speckle photography illuminated by laser-beam-sweeps which not only gives images of flow patter, but two-dimensional velocity distributions. The results of visualization give clear understanding of the change of pressure rise tendency in the region. The flow pattern drastically changes as the Reynolds number exceeds 10. The vortex street developed from two shear layers bounding reverse flow generated in the convergent region of the peristaltic wave appears.