  An extension of the Ambegaokar-Baratoff relation to a
superconductor-insulator-superconductor (SIS) Josephson junction with multiple
tunneling channels is derived. Appling the resultant relation to a SIS
Josephson junction formed by an iron-based (five-band) and a single-band
Bardeen-Cooper-Schrieffer (BCS) type superconductors, a theoretical bound of
the Josephson critical current ($I_{\rm c}$) multiplied by the resistance of
the junction ($R_{\rm n}$) is given. We reveal that such a bound is useful for
identifying the pairing symmetry of iron-pnictide superconductors. One finds
that if a measured value of $I_{\rm c}R_{\rm n}$ is smaller than the bound then
the symmetry is $\pm s$-wave, and otherwise $s$-wave without any sign changes.
In addition, we stress that temperature dependence of $I_{\rm c}R_{\rm n}$ is
sensitive to the difference of the gap functions from the BCS type gap formula
in the above heterojunction.
