  If a real scalar field is dominated by non-relativistic modes, then it
approximately conserves its particle number and obeys an equation that governs
a complex scalar field theory with a conserved global U(1) symmetry. From this
fact, it is shown that the I-ball (oscillon) can be naturally understood as a
projection (e.g., real part) of the non-relativistic Q-ball solution. In
particular, we clarify that the stability of the I-ball is guaranteed by the
U(1) symmetry in the corresponding complex scalar field theory as long as the
non-relativistic condition holds. We also discuss the longevity of I-ball from
the perspective of the complex scalar field in terms of U(1) charge violating
processes.
