A control device for an additive supply system, wherein the additive supply system includes: a tank that stores an additive in a liquid form; an addition valve through which the additive in the tank is supplied by injection to a catalyst provided in an exhaust passage of an internal combustion engine; a main passage through which the additive in the tank is supplied to the addition valve; a pump provided in the main passage, wherein the pump feeds the additive under pressure; a return passage branching off the main passage to be connected to the tank; and a check valve provided in the return passage, wherein the check valve prevents flow of the additive from the return passage into the main passage, and the check valve is opened if a pressure in the main passage becomes higher than or equal to a predetermined valve opening pressure, wherein the control device is configured to be capable of executing retrieval control of drawing the additive in the main passage back into the tank by rotating the pump in reverse, characterized in that, when executing filling control of filling the main passage with the additive after execution of the retrieval control, the control device rotates the pump forward while keeping the addition valve closed, and then opens the addition valve and rotates the pump forward, and when the control device executes the filling control, the lower the range within which the pressure in the main passage changes, the longer becomes the period in which the control device drives the pump to rotate forward while the addition valve is kept open after the pump rotates forward while the addition valve is kept closed. a tank that stores an additive in a liquid form; an addition valve through which the additive in the tank is supplied by injection to a catalyst provided in an exhaust passage of an internal combustion engine; a main passage through which the additive in the tank is supplied to the addition valve; a pump provided in the main passage, wherein the pump feeds the additive under pressure; a return passage branching off the main passage to be connected to the tank; and a check valve provided in the return passage, wherein the check valve prevents flow of the additive from the return passage into the main passage, and the check valve is opened if a pressure in the main passage becomes higher than or equal to a predetermined valve opening pressure, wherein the control device is configured to be capable of executing retrieval control of drawing the additive in the main passage back into the tank by rotating the pump in reverse, characterized in that, when executing filling control of filling the main passage with the additive after execution of the retrieval control, the control device rotates the pump forward while keeping the addition valve closed, and then opens the addition valve and rotates the pump forward, and when the control device executes the filling control, the lower the range within which the pressure in the main passage changes, the longer becomes the period in which the control device drives the pump to rotate forward while the addition valve is kept open after the pump rotates forward while the addition valve is kept closed.