An approximate expression for the (3.3) partial wave amplitude is given from the analyticity, unitarity, and threshold behaviuor consideration, in which only two parameters, namely the coupling constant and resonance level, are introduced. This expression is in good agreement with the experimental data from the threshold up to the pion laboratory kinetic energy of about 350 MeV. In comparison with Chew-Low's result, it gives the variation of q3/ω ctgδ33 with ω not following a straight line but a curve concave downward. Such an improvement is just required by the experimental data. It is shown, there is some inadequacy concerning analyticity in the Chew-Low's expansion, and this inadequacy has been overcomed in the present formulation.