The pinching effect of the magnetic field is studied as a possible mechanism for collimating stellar winds into bipolar flows in star-forming regions. Axisymmetric, steady, polytropic stellar wind models are constructed by solving numerically the magnetohydronynamic equation. The magnetic field is assumed to be radial near the star. Far from the star the magnetic field makes a spiral near the equatorial plane and the toroidal components build up. The pressure and the tension of the toroidal magnetic field deflects the wind away from the equator toward the rotation axis. This deflection continues until the flow becomes nearly cylindrical near the rotation axis. This state is a pinch configuration in which dense gas is confined into a collimated flow by the toroidal magnetic field. This mechanism works under very general circumstances in the magnetic stellar winds from rotating stars.