1. A method for optimizing gray level value of ultrasonic images used in ultrasonic system, comprises: an acquiring step for acquiring the ultrasonic image; a searching step for searching non-evenly divided sub-areas from the acquired ultrasonic image; analyzing step for analyzing the change of gray level in each of the sub-areas in the direction of depth; and calculating step for calculating an optimized gray level value of the ultrasonic image based on the result of the analyzing step.