An automated teller machine, comprising: a main body; a deposit/withdrawal unit provided at one side in the main body and configured to separate and stack a medium; a customer service unit provided at a front side of the main body and configured to receive a medium from a user; and a tray unit configured to transfer a medium between the deposit/withdrawal unit and the customer service unit while reciprocating therebetween, wherein the deposit/withdrawal unit includes: a bill stopper that is rotatably provided to open and close an opening of the deposit/withdrawal unit through which the tray unit passes; and a link unit configured to open and close the bill stopper by providing a rotational force to the bill stopper, wherein the link unit includes: a rotatable driving wheel; a fixed frame having one end fixedly coupled to one end of the bill stopper and rotating together with the bill stopper; and a movable link having opposite ends, one end being hinge-coupled to the other end of the fixed frame and the other end being hinge-coupled to a circumference of the driving wheel, and wherein in the link unit, when the opening of the deposit/withdrawal unit is completely opened by the rotation of the bill stopper, the opposite ends of the movable link and a center of rotation of the driving wheel are positioned on a straight line. a main body; a deposit/withdrawal unit provided at one side in the main body and configured to separate and stack a medium; a customer service unit provided at a front side of the main body and configured to receive a medium from a user; and a tray unit configured to transfer a medium between the deposit/withdrawal unit and the customer service unit while reciprocating therebetween, wherein the deposit/withdrawal unit includes: a bill stopper that is rotatably provided to open and close an opening of the deposit/withdrawal unit through which the tray unit passes; and a link unit configured to open and close the bill stopper by providing a rotational force to the bill stopper, a bill stopper that is rotatably provided to open and close an opening of the deposit/withdrawal unit through which the tray unit passes; and a link unit configured to open and close the bill stopper by providing a rotational force to the bill stopper, wherein the link unit includes: a rotatable driving wheel; a fixed frame having one end fixedly coupled to one end of the bill stopper and rotating together with the bill stopper; and a movable link having opposite ends, one end being hinge-coupled to the other end of the fixed frame and the other end being hinge-coupled to a circumference of the driving wheel, and a rotatable driving wheel; a fixed frame having one end fixedly coupled to one end of the bill stopper and rotating together with the bill stopper; and a movable link having opposite ends, one end being hinge-coupled to the other end of the fixed frame and the other end being hinge-coupled to a circumference of the driving wheel, and wherein in the link unit, when the opening of the deposit/withdrawal unit is completely opened by the rotation of the bill stopper, the opposite ends of the movable link and a center of rotation of the driving wheel are positioned on a straight line.