  We prove that for a certain class of kernels $K(y)$ that viscosity solutions
of the integro-differential equation $$
  \int_{\mathbb R^n} (u(x+y) - 2 u(x) + u(x-y)) K(y) dy = f(x,u(x)) $$ are
locally analytic if $f$ is an analytic function. This extends the result of
Albanese, Fiscella, Valdinoci that such solutions belong to certain Gevrey
classes.
