  It is demonstrated theoretically and experimentally that the low energy
density of states $N(E)$ is described by a singular V-shape form
$N(E)=N_0(H)+\alpha |E|+O(E^2)$ for all clean superconductors in a vortex
state, irrespective of the underlying gap structure. The linear term $\alpha
|E|$ which has been not recognized so far is obtained by exactly evaluating the
vortex contribution. Based on microscopic Eilenberger theory $N(E)$ is
evaluated for isotropic gap, line and point node gaps to yield a V-shape
$N(E)$. STS-STM experiments on NbSe$_2$ and YNi$_2$B$_2$C give direct evidence
for this. We provide arguments on significance of this finding and on relevance
to other experiments.
