  We present a fluctuation relation for heat dissipation in a nonequilibrium
system. A nonequilibrium work is known to obey the fluctuation theorem in any
time interval $t$. A heat, which differs from a work by an energy change, is
shown to satisfy a modified fluctuation relation. Modification is brought by
correlation between a heat and an energy change during nonequilibrium processes
whose effect may not be negligible even in the $t\to\infty$ limit. The
fluctuation relation is derived for overdamped Langevin equation systems, and
tested in a linear diffusion system.
