  We investigate the Q-ball decay into the axino dark matter in the
gauge-mediated supersymmetry breaking. In our scenario, the Q ball decays
mainly into nucleons and partially into axinos to account for the baryon
asymmetry and the dark matter of the universe simultaneously. The Q ball decays
well before the big bang nucleosynthesis so that it is not affected by the
decay. The decay into the supersymmetric particles of the minimal
supersymmetric standard model is kinematically prohibited until the very end of
the decay, and we could safely make their abundances small enough for the
successful big bang nucleosynthesis. We show the regions of axino model
parameters and the Q-ball parameters which realize this scenario.
