The conventional Rotman lens antenna using a waveguide lens has the bended waveguide from the inner lens contour to the antenna element. However in this paper the improved Rotman lens antenna having a straight waveguide is proposed. By applying this technique, the beam steering antenna for BMWS application which has 3 input ports and 6 antenna elements consisted of open-ended waveguide has been designed and fabricated. From the measurement, the gain of the beam steering antenna is 15.4 ~ 17.2 dB. The direction angle is 0$^{circ}$, +14$^{circ}$~ +16$^{circ}$, -15$^{circ}$~ -16$^{circ}$ and the side lobe level is -l0 ~ 20 dB.