This paper describes on elucidation of the mechanism for underwater locomotion of a neutrophil toward the inflammation part. In our previous investigations, there are oscillation of concentration gradient on the membrane of neutrophil when it is moving toward the direction of higher concentration. It is necessary to find out the reason why the neutrophil is continuing to move under the concentration gradient. In this paper, the mathematical model of driving force by surface tension gradient. To establish the model, it is assumed that the surface tension depends on the concentration and temperature. It is found that the velocity by this oscillation depend on the amplitude of concentration gradient and period of oscillation. In this investigation, the mathematical model of driving force by oscillation of concentration gradient was established, and it will be confirmed by the experiment of microscopic observation of concentration gradient.