  Magnetic susceptibility and the magnetization process have been measured in
\green polycrystal. In this compound, the magnetic manganese ion exists as
Mn$^{5+}$ in a tetrahedral environment, and thus the magnetic interaction can
be described by an S=1 Heisenberg model. The ground state was found to be a
spin singlet with an excitation gap $\Delta/k_{\rm B}=11.2$ K. Magnetization
plateaus were observed at zero and at half of the saturation magnetization.
These results indicate that the present system can be represented by a coupled
antiferromagnetic dimer model.
