  We derive a new non-abelian Stokes theorem by rewriting the Wilson loop as a
gauge-invariant area integral, at the price of integrating over an auxiliary
field from the coset SU(N) / [U(1)]^{N-1} space. We then introduce the
relativistic quark--monopole interaction as a Wess--Zumino-type action, and
extend it to the non-abelian case. We show that condensation of monopoles and
confinement can be investigated in terms of the behaviour of the monopole world
lines. One can thus avoid hard problems of how to introduce monopole fields and
dual Yang--Mills potentials.
