  We study the effect of irradiation from two accretion disks (minidisks)
around respective black holes of stellar to intermediate masses in a circular
binary on the spectrum of a circumbinary disk (CBD) surrounding them. We assume
the CBD to be a standard disk and adopt the orbit-averaged irradiation flux
because the viscous timescale is much longer than the orbital period. We then
solve the energy equation both analytically and numerically to compute the CBD
temperature distribution and the corresponding disk spectrum. We find that the
analytically calculated spectra are in good agreement with the numerical ones.
The CBD spectrum is almost independent of the binary mass ratio. We also find
that the combined spectra of two minidisks and the CBD have double peaks, one
peak in the soft X-ray band and the other in the infrared (IR) band. The former
peak comes from the two minidisks, while the latter peak from the CBD. The
observed flux density increases with frequency as $\nu^{1/3}$ towards the soft
X-ray peak, while it decreases with frequency away from the IR peak as
$\nu^{-5/3}$. The latter feature is testable with near-IR observations with
Subaru and JWST.
