  We demonstrate that a Jackiw-Nohl-Rebbi solution as the most general
two-instanton generates a circular loop of magnetic monopole in
four-dimensional Euclidean SU(2) Yang-Mills theory, in contrast to the
one-instanton solution in the regular gauge for which no such magnetic monopole
loops exist. These results together with our previous result indicate that
two-instanton solution and two-meron solution with the same asymptotic behavior
in the long distance are responsible for quark confinement based on the dual
superconductivity picture.
