  The $S=1/2$ antiferromagnetic Heisenberg model on multi-leg ladders is
investigated. Criticality of the ground-state transition is explored by means
of finite-size scaling. The ladders with an even number of legs and those with
an odd number of legs are distinguished clearly. In the former, the energy gap
opens up as $\Delta E\sim{J_\perp}$, where ${J_\perp}$ is the strength of the
antiferromagnetic inter-chain coupling. In the latter, the critical phase with
the central charge $c=1$ extends over the whole region of ${J_\perp}>0$.
