To explain the mechanism of the shock formation in a simple shock tube, the multi-stage model was presented, which improved White's theory, and some numerical calculations were carried out. Simple and White's theories correspond respectively to the case of zero or one stage of the present multi-stage model. The shock Mach number calculated by the multi-stage model is always larger than that obtained by the White's model. The difference between them becomes large as the diaphragm pressure ratio increases. Further in order to express the variation of the shock Mach number along the tube a function II (x) was proposed. By using this function, the phenomenon that shocks stronger than that predicted by the simple theory would possibly happen was discussed.