  We show that the magnetic monopole promoted to the dyon due to the vacuum
angle $\theta$ resolves the U(1) problem in the sense that the dyon obtained in
this way gives a dominant contribution to the topological susceptibility. For
this purpose, we derive an Abelian-projected effective gauge theory written in
terms of Abelian degrees of freedom, which is obtained by integrating out all
the off-diagonal degrees of freedom involved in the SU(2) Yang-Mills theory
with the vacuum angle $\theta$. We evaluate the topological susceptibility by
estimating the classical part of the effective dyon action obtained by
performing the duality transformation. The obtained result is consistent with
the Veneziano--Witten formula.
