  We compare the nonthermal emission from clusters of galaxies undergoing minor
mergers (``accreting'' clusters) and major mergers (``merging'' clusters). For
accreting clusters, the radial distribution of the nonthermal emission in the
clusters is also calculated. The relativistic electrons, which are the origin
of the nonthermal radiation through inverse Compton (IC) and synchrotron
mission, are assumed to be accelerated at shocks produced by accretion or
mergers. We estimate the typical accretion rate and merger probability
according to a hierarchical clustering model. We show that the total luminosity
of IC emission from accreting and merging clusters are similar. On the other
hand, the luminosity of synchrotron radio emission of the former is much
smaller than that of the latter. We show that about 10% of clusters at z~0
should have hard X-ray and radio nonthermal emissions due to their last major
merger, which are comparable to or dominate those due to ongoing accretion.
Moreover, 20-40% of clusters should have significant EUV emission due to their
last merger. We also investigate the case where the criterion of mergers is
relaxed. If we extend the definition of a merger to an increase in the mass of
the larger subcluster by at least 10% of its initial mass, about 20-30% of
clusters at z~0 should have hard X-ray and radio nonthermal emissions due to
the merger even in a low density universe. We compare the results with
observations. We find that the observed EUV emission from clusters is not
attributed to accretion. If the diffuse radio emission observed in clusters is
synchrotron emission from electrons accelerated via accretion or merging, the
magnetic fields of clusters are generally as small as ~0.1 mu G.
