  We investigate the instability of the unstable circular orbit of a charged
null particle to test the strong cosmic censorship conjecture in Nariai-type
near-extremal Reissner--Nordstr\"{o}m--de Sitter black holes. The instability
is estimated as the Lyapunov exponent and found to depend on the mass and
charge of the black hole. Then, we explicitly show that charged null particles
in unstable circular orbits correspond to the charged massless scalar field in
the eikonal limit. This provides a compact relationship representing the
quasinormal frequency in terms of the characteristics of unstable circular
orbits in near Nariai-type extremal conditions. According to this relationship,
the strong cosmic censorship conjecture is valid.
