  We clarify that the antiferromagnetic order in the distorted tetrahedral
quasi-one dimensional spin system induces electric polarizations. In this
system, the effects of the low dimensionality and the magnetic frustration are
expected to appear simultaneously. We obtain the magnetic-field-temperature
phase diagram in Cu3Mo2O9 by studying the dielectric constant and the
spontaneous electric polarization. Around the tricritical point at 10 T and 8
K, the change of the direction in the electric polarization causes a colossal
magnetocapacitance. We calculate the charge redistribution in the small spin
cluster consisting of two magnetic tetrahedra to demonstrate the electric
polarization induced by the antiferromagnetism.
