A shutter unit 17 of a bill validator comprises a blocking member 18 movable between an interceptive position for closing the through hole 74 of the shutter unit 17 and a release position away from the through hole 74 for allowing passage of the bill through the through hole 74 into a slot 13 of the validating unit 20; a control unit 48 movable between an inactive position for barring movement of the blocking member 18 to the release position and an operative position for allowing movement of the blocking member 18 to the release position; and an actuator 50 for shifting the control unit 48 between the inactive and operative positions. When the blocking member 18 is forcibly moved to the release position in an unauthorized manner, the control unit 48 in the inactive position effectively inhibits and resists movement of the blocking member 18 to the release position.