We focus on a target position estimation algorithm to detect targets by using an ultrasonic sensor array equipped in front of a vehicle. Conventional method estimates target positions probabilistically by using multilateration based on target existence probability. However, the conventional multilateration has a problem that causes estimation error in the direction of the arc drawn by the measuring sensors in case of measuring by using a biased sensors layout such as one side observation. Therefore, we propose novel position estimation applying a weighting function based on TDOA (Time Difference of Arrival) value to the conventional method. The effectiveness of the proposed method is confirmed through numerical examples by computer simulation.