In order to obtain information on the structural geometry of $NH_4HSeO_4$ near the phase transition temperature, the spectrum and spin-lattice relaxation time in the rotating frame $T_{1{ ho}}$ for the ammonium and hydrogen-bond protons were investigated through $^1H$ MAS NMR. $T_{1{ ho}}$ for the hydrogen-bond protons abruptly decreased at high temperature and it is associated with the change in the structural geometry in $O-H{cdots}O$ bonds. This mobility of the hydrogen-bond protons may be the main reason for the high conductivity.