Abstract Measurements of 1 H-NMR and 77 Se-NMR have been conducted with CsHSeO 4 , in order to clarify dynamics of proton and SeO 4 tetrahedron which is related to transport of proton below and above T c = 401 K. An NMR absorption line is observed in 1 H-NMR measurement and its line width decreases with increasing temperature below T c , due to the motional narrowing. These results indicate that the mobile proton exists even below T c . A correlation time τ is estimated from the line width for the mobile proton to be about 10 − 4 –10 − 6 s below T c . Furthermore it is deduced from 77 Se-NMR measurements that the reorientational motion of SeO 4 tetrahedron is related to the transport of proton below T c .