The non-linear saturation theory of tearing modes is applied to tokamak plasmas produced by an appropriate combination of gas puffing and plasma current rise in JIPP T-II. The calculated relative amplitude of poloidal-field fluctuations is well correlated with the experimental data. The radial extent of the island widths of m/n = 2/1 and 3/2 modes which are derived in the calculation suggests that the 'soft' major disruption, where the plasma current suffers only a slight change, can be triggered either by the overlapping of a 2/1 with a 3/2 island or by the contact of a 2/1 island with a limiter. Moreover, it is consistent with the experimental data that the 'hard' major disruption, which brings about an abrupt and significant reduction in the plasma current, results from the combined effect of the two abovementioned causes for soft major disruption.