  We study the regular surface defect in the $\Omega$-deformed four dimensional
supersymmetric gauge theory with gauge group $SU(N)$ with $2N$ hypermultiplets
in fundamental representation. We prove its vacuum expectation value obeys the
Knizhnik-Zamolodchikov equation for the $4$-point conformal block of the
$\widehat{\mathfrak{sl}}_{N}$-current algebra, originally introduced in the
context of two dimensional conformal field theory. The level and the vertex
operators are determined by the parameters of the $\Omega$-background and the
masses of the hypermultiplets, the cross-ratio of the $4$ points is determined
by the complexified gauge coupling. We clarify that in a somewhat subtle way
the branching rule is parametrized by the Coulomb moduli. This is an example of
the BPS/CFT relation.
