  We consider the regularity of stationary solutions to the linearized
Boltzmann equations in bounded $C^1$ convex domains in $\mathbb{R}^3$ for gases
with cutoff hard potential and cutoff Maxwellian gases. We prove that the
stationary solutions solutions are H\"{o}lder continuous with order
$\frac1{2}^-$ away from the boundary provided the incoming data have the same
regularity. The key idea is to partially transfer the regularity in velocity
obtained by collision to space through transport and collision.
