  We perform canonical quantization of open strings in the $D$-brane background
with a $B$-field. Treating the mixed boundary condition as a primary
constraint, we get a set of secondary constraints. Then these constraints are
shown to be equivalent to orbifold conditions to be imposed on normal string
modes. These orbifold conditions are a generalization of the familiar orbifold
conditions which arise when we describe open strings in terms of closed
strings. Solving the constraints explicitly, we obtain a simple Hamiltonian for
the open string, which reveals the nature of noncommutativity transparently.
