We propose a new technique which can treat low viscous viscoelastic fluids and which can measure the start-up and relaxation behavior of elongational stress in step elongation rate flow. This new device consists of two parts: the squeeze cell and the optical analyzer. The squeeze cell generates the planar squeeze flow and causes the step planar elongational flow at the stagnation point. The optical system measures the transient behavior of both birefringence and orientation angle in the start-up and relaxation region at the stagnation point. In this paper, we demonstrate the two-dimensionality of the flow in the planar squeeze cell. Next, we evaluated the planar elongational stress and viscosity from optical results for Maxwell fluid. These results show good agreement with Maxwell model. Finally, we clarify the measurement accuracy from the measurement results in channels of various sizes.