  For any positive integer $N$, we completely determine the structure of the
rational cuspidal divisor class group of $X_0(N)$, which is conjecturally equal
to the rational torsion subgroup of $J_0(N)$. More specifically, for a given
prime $\ell$, we construct a rational cuspidal divisor $Z_\ell(d)$ for any
non-trivial divisor $d$ of $N$. Also, we compute the order of the linear
equivalence class of the divisor $Z_\ell(d)$ and show that the $\ell$-primary
subgroup of the rational cuspidal divisor class group of $X_0(N)$ is isomorphic
to the direct sum of the cyclic subgroups generated by the linear equivalence
classes of the divisors $Z_\ell(d)$.
