The present invention provides an alloy fine particle including palladium and ruthenium, the alloy fine particle including at least one first phase in which the palladium is more abundant than the ruthenium and at least one second phase in which the ruthenium is more abundant than the palladium, the at least one first phase and the at least one second phase being separated by a phase boundary, the palladium and the ruthenium being distributed in the phase boundary in such a manner that the molar ratio of the palladium and the ruthenium continually changes, a plurality of crystalline structures being present together in the phase boundary.