An improved pulse shaper is proposed which is able to control both the spatial and temporal profile of femtosecond light pulses. Our pulse shaper exploits the spatio-temporal coupling effect seen in pulse shapers. Its properties are numerically analyzed by application of the Wigner distribution function. We confirm that the spatio-temporal output pulse track dictates the differentiation of the phase mask; that the degree of spatio-temporal coupling is determined by the focal length ratio of the lenses in the pulse shaper; and that space to spatial-frequency chirp results from misalignment of lenses.