  The neutron electric dipole moment is investigated in a supersymmetric
electroweak model admitting $ {\rm SU(2)}_W $ singlet and $ Q = -1/3 $, say $ D
$-type, quarks. Significant $ CP $ violating phases appear in the coupling
between the singlet $ D $-type quarks and the ordinary $ d $-type quarks. Then,
through the $ d $-$ D $ mixing effects on the squark mass terms the gluino
one-loop diagrams provide naturally the contributions $ \sim 10^{-26} e~{\rm
cm} $ comparable to the current experimental bound on the neutron electric
dipole moment. The $ CP $ violation in the $ d $-$ D $ coupling would also be
relevant for the electroweak baryogenesis.
