  Unconventional superconductivity often competes or coexists with other
electronic orders. In iron-based superconductors, relationship between
superconductivity and the nematic state, where the lattice rotational symmetry
is spontaneously broken in the electronic states, has been discussed but
unclear. Using spectroscopic-imaging scanning tunneling microscopy, we
investigate how the band structure and the superconducting gap evolve in
FeSe$_{1-x}$S$_x$, as the sulfur substitution suppresses nematicity that
eventually diminishes at the nematic end point (NEP) at $x=0.17$. Anisotropic
quasiparticle-interference patterns, which represent the nematic band
structure, gradually become isotropic with increasing $x$ without detectable
anomalies in the band parameters at the NEP. By contrast, the superconducting
gap, which is almost intact in the nematic phase, suddenly shrinks as soon as
$x$ exceeds the NEP. Our observation implies that the presence or absence of
nematicity results in two distinct pairing states, whereas the pairing
interaction is insensitive to the strength of nematicity. This provides a clue
for understanding the pairing mechanism.
