  We study the dynamics of multi-junction switching (MJS): several intrinsic
Josephson junctions (IJJs) in an array switch to the finite voltage state
simultaneously. The number of multi-switching junctions ($N$) was successfully
tuned by changing the load resistance serially connected to an
Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta}$ IJJ array. The independence of the
escape rates of $N$ in the macroscopic quantum tunneling regime indicates that
MJS is a $successive$ switching process rather than a $collective$ process. The
origin of MJS is explained by the gradient of a load curve and the relative
magnitudes of the switching currents of quasiparticle branches in the
current-voltage plane.
