We have run a Direct Numerical Simulation of a spatially developing shear mixing layer. The aim of this study is to clarify the influence of the large-scale structure on the turbulent Prandtl number Pr_T. As a main conclusion, Pr_T takes a small value (Pr_T ~ 0.5) in the dominant region of the large-scale structure. The budget analyses for the Reynolds stress equation and the scalar flux equation revealed that the differences between the momentum and scalar transfer are caused by terms related to pressure (i.e., pressure-strain correlation term, pressure-scalar gradient correlation term, and pressure diffusion terms). Phenomenally, the momentum in the field where a large-scale vortex coexists tends to be transported toward the counter-gradient direction under the influence of pressure, but the scalar is transported toward the gradient direction. As a result, it is thought that the difference in the driving force between the momentum and scalar transport causes the decrease of the Pr_T.