  We consider the problem of geometric optimization for the lowest eigenvalue
of the two-dimensional Schr\"odinger operator with an attractive
$\delta'$-interaction of a fixed strength, the support of which is a
$C^2$-smooth contour. Under the constraint of a fixed length of the contour, we
prove that the lowest eigenvalue is maximized by the circle. The proof relies
on the min-max principle and the method of parallel coordinates.
