  We calculate how much a first-quantized string is excited after crossing the
inner horizon of charged Vaidya solutions, as a simple model of generic black
holes. To quantize a string suitably, we first show that the metric is
approximated by a {\it plane-wave} metric near the inner horizon when the
surface gravity of the horizon $\kappa_I$ is small enough. Next, it is
analytically shown that the string crossing the inner horizon is excited
infinitely in an asymptotically flat spacetime, while it is finite in an
asymptotically de Sitter spacetime and the string can pass across the inner
horizon when $\kappa_I<2\kappa:= 2 {min}\{\kappa_B,\kappa_C \}$, where
$\kappa_B$~($\kappa_C$) is the surface gravity of the black hole~(cosmological)
event horizon. This implies that the strong cosmic censorship holds in an
asymptotically flat spacetime, while it is violated in an asymptotically de
Sitter spacetime from the point of view of string theory.
