An attachment coupler for heavy machinery, which is capable of detachably coupling an attachment to an arm of the heavy machinery, comprising: a coupler body configured to be coupled to the arm of the heavy machinery; a fixed hook configured to be formed on the coupler body and coupled to the attachment via a first coupling pin of the attachment which is inserted thereinto; a movable hook configured to be rotatably coupled to the coupler body via a hinge axis and coupled to the attachment via a second coupling pin which is inserted thereinto; a hydraulic cylinder configured to rotate the movable hook to be coupled to or disengaged from the second coupling pin; and a locking device configured to comprise a locking hook and an engagement device, wherein the locking hook is configured to be rotatably coupled to the coupler body via a hinge axis and close an open end of the fixed hook while being coupled to the first coupling pin and the engagement device is configured to rotate in association with the rotation of the movable hook to allow the locking hook to be coupled to or disengaged from the first coupling pin in order that the first coupling pin is prevented from being disengaged from the fixed hook while the movable hook stays being coupled to the second coupling pin, wherein the engagement device comprises a link member, characterized in that, the link member is configured to be disposed between the locking hook and the movable hook and coupled to the coupler body via a hinge axis and to comprise one end facing the movable hook and the other end coupled to the locking hook via a hinge axis, as the movable hook rotates in a direction of being disengaged from the second coupling pin, the one end of the link member moves back by the movable hook and thus rotates the locking hook in a direction allowing the locking hook to be disengaged from the first coupling pin, and as the movable hook rotates in a direction of being coupled to the second coupling pin, the one end of the link member moves away from the movable hook and thereby allows the locking hook to rotate by its own weight and thus be coupled to the first coupling pin. a coupler body configured to be coupled to the arm of the heavy machinery; a fixed hook configured to be formed on the coupler body and coupled to the attachment via a first coupling pin of the attachment which is inserted thereinto; a movable hook configured to be rotatably coupled to the coupler body via a hinge axis and coupled to the attachment via a second coupling pin which is inserted thereinto; a hydraulic cylinder configured to rotate the movable hook to be coupled to or disengaged from the second coupling pin; and a locking device configured to comprise a locking hook and an engagement device, wherein the locking hook is configured to be rotatably coupled to the coupler body via a hinge axis and close an open end of the fixed hook while being coupled to the first coupling pin and the engagement device is configured to rotate in association with the rotation of the movable hook to allow the locking hook to be coupled to or disengaged from the first coupling pin in order that the first coupling pin is prevented from being disengaged from the fixed hook while the movable hook stays being coupled to the second coupling pin, wherein the engagement device comprises a link member, characterized in that, the link member is configured to be disposed between the locking hook and the movable hook and coupled to the coupler body via a hinge axis and to comprise one end facing the movable hook and the other end coupled to the locking hook via a hinge axis, as the movable hook rotates in a direction of being disengaged from the second coupling pin, the one end of the link member moves back by the movable hook and thus rotates the locking hook in a direction allowing the locking hook to be disengaged from the first coupling pin, and as the movable hook rotates in a direction of being coupled to the second coupling pin, the one end of the link member moves away from the movable hook and thereby allows the locking hook to rotate by its own weight and thus be coupled to the first coupling pin.