  We find unusual temperature dependence of the ferromagnetic resonance (FMR)
frequency $f_{\rm R}$ for the spin-reorientation (SR) transition, in which the
easy axis changes depending on temperature, observed in the Nd permanent
magnet, Nd$_2$Fe$_{14}$B: $f_{\rm R} \sim 0$ below the SR transition
temperature ($T_{\rm R}$), drastic increase of $f_{\rm R}$ around $T_{\rm R}$,
and decrease from a peak at higher temperatures. It is nontrivial that the SR
transition causes the unusual behavior of the FMR frequency in a wide
temperature region. We show the mechanism of the temperature dependence by
theoretical and computational analyses. We derive a general relation between
$f_{\rm R}$ and magnetizations to help the understanding of the mechanism, and
clarify that the fluctuation of the transverse magnetization is a key
ingredient for the resonance in all temperature regions.
