  We search for dimuon decays of a low mass particle in the decays B0->K*0 X
and B0->rho X using a data sample of 657x10^6 B Bbar events collected with the
Belle detector at the KEKB asymmetric-energy e^+ e^- collider. We find no
evidence for such a particle in the mass range from 212 MeV/c2 to 300 MeV/c2,
and set upper limits on its branching fractions. In particular, we search for a
particle with a mass of 214.3 MeV/c2 reported by the HyperCP experiment, and
obtain upper limits on the products B(B0->K*0 X)xB(X->mu^+ mu^-) < 2.26
(2.27)x10^-8 and B(B0->rho0 X)xB(X->mu^+ mu^-) < 1.73 (1.73)x10^-8 at 90% C.L.
for a scalar (vector) X particle.
