  We study Dirac commutators of canonical variables on D-branes with a constant
Neveu-Schwarz 2-form field by using the Dirac constraint quantization method,
and point out some subtleties appearing in previous works in analyzing
constraint structure of the brane system. Overcoming some ad hoc procedures, we
obtain desirable noncommutative coordinates exactly compatible with the result
of the conformal field theory in recent literatures. Furthermore, we find
interesting commutator relations of other canonical variables.
