  We measured the temperature dependences of the static magnetization and the
spin excitation in a square-lattice multiferroics Ba$_2$MnGe$_2$O$_7$. An
anisotropy gap of the observed low energy mode is scaled by electric
polarization rather than a power of sublattice moment. Spin nematic interaction
in effective spin Hamiltonian, which is equivalent to interaction of electric
polarization, is responsible for the easy-axis anisotropy. The nontrivial
behavior of the anisotropy gap can be rationalized as change of the hybridized
$d$-$p$ orbital with temperature, leading to the temperature dependence of the
spin nematic interaction.
