  I-balls/oscillons are long-lived and spatially localized solutions of real
scalar fields. They are produced in various contexts of the early universe in,
such as, the inflaton evolution and the axion evolution. However, their decay
process has long been unclear. In this paper, we derive an analytic formula of
the decay rate of the I-balls/oscillons within the classical field theory. In
our approach, we calculate the Poynting vector of the perturbation around the
I-ball/oscillon profile by solving a relativistic field equation, with which
the decay rate of the I-ball/oscillon is obtained. We also perform a classical
lattice simulation and confirm the validity of our analytical formula of the
decay rate numerically.
