  We consider the Steklov zeta function $\zeta$ $\Omega$ of a smooth bounded
simply connected planar domain $\Omega$ $\subset$ R 2 of perimeter 2$\pi$. We
provide a first variation formula for $\zeta$ $\Omega$ under a smooth
deformation of the domain. On the base of the formula, we prove that, for every
s $\in$ (--1, 0) $\cup$ (0, 1), the difference $\zeta$ $\Omega$ (s) -- 2$\zeta$
R (s) is non-negative and is equal to zero if and only if $\Omega$ is a round
disk ($\zeta$ R is the classical Riemann zeta function). Our approach gives
also an alternative proof of the inequality $\zeta$ $\Omega$ (s) -- 2$\zeta$ R
(s) $\ge$ 0 for s $\in$ (--$\infty$, --1] $\cup$ (1, $\infty$); the latter fact
was proved in our previous paper [2018] in a different way. We also provide an
alternative proof of the equality $\zeta$ $\Omega$ (0) = 2$\zeta$ R (0)
obtained by Edward and Wu [1991].
