A banknote handling device includes a safe that protects banknotes (casing of a second unit), a first conveyance path externally provided to the safe, and a second conveyance path internally provided to the safe. The safe is provided with at least one opening portion through which the banknotes pass. A passing unit including a passing conveyance mechanism that passes a banknote between the first conveyance path and the second conveyance path and a left-behind sensor that detects a banknote that is left-behind, is detachably installed inside the opening portion of the safe.