We have investigated a mechanism for the switching of stacked intrinsic Josephson junctions in current-voltage characteristics. In a current-biased measurement, collective switching has been observed, while in a voltage-biased measurement, clear branching structure has been observed. In the voltage-biased measurement, the interaction between different intrinsic junctions has clearly appeared in decrease of values of Josephson critical current (I c ) with increasing the number of junctions in the resistive state. The I c decrease leading to the collective behavior has been explained by taking effective heat diffusion into account.