We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable ($Delta$ $sim$ 11 meV) to that of the inner electron and hole pockets ($sim$12 meV), although it is substantially larger than that of the outer hole pocket ($sim$6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with $Delta$($k$) = $Delta$$_0$cos$k_x$cos$k_y$ formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.
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