A direct numerical simulation of a temporally developing mixing layer with a passive scalar transport is performed for various Schmidt numbers (Sc = 0.25, 1, 4, and 8). Turbulent mixing is investigated near the turbulent/non-turbulent interface (TNTI), which is a layer consisting of the turbulent sublayer (TSL) and viscous superlayer (VSL). The irrotational boundary, which is close to the outer edge of the TNTI layer, is detected as the isosurface of small vorticity magnitude. The movement of fluid elements relative to the irrotational boundary movement is analyzed. Once the non-turbulent fluid is entrained into the VSL across the irrotational boundary by the viscous diffusion of vorticity, the fluid moves away from the irrotational boundary in the VSL in the normal direction of the irrotational boundary. After the fluid reaches the TSL, it is transported in the tangential direction of the irrotational boundary and is mixed with the fluid coming from the turbulent core (TC) region. The boundary between the TSL and VSL roughly separates the region (VSL) mostly consisting of the fluid entrained from the non-turbulent flow from the region (TSL) where the fluids from both the TC and non-turbulent regions coexist. Therefore, the scalar value in the VSL is close to the non-turbulent value especially for high Sc cases. Because of a large difference in the scalar between the TSL and VSL, a peak value of the conditional mean scalar dissipation rate appears near the boundary between the TSL and VSL independently of Sc.