  We can directly investigate the ground state in magnetization-plateau fields
(plateau ground state) using neutron diffraction measurements. We performed
neutron diffraction measurements on the spin-5/2 trimer substance
SrMn$_3$P$_4$O$_{14}$ in magnetization-plateau fields. The integrated
intensities of magnetic reflections calculated using an expectation value of
each spin in a plateau ground state of an isolated-trimer model agree well with
those obtained experimentally in the magnetization-plateau fields. We succeeded
in direct observation of a plateau ground state in SrMn$_3$P$_4$O$_{14}$.
