The objective of the present invention is to provide a controller for a supercharger-equipped internal combustion engine and a control method that can reduce man-hours for data measurement and matching, which are required to perform while the internal combustion engine and the supercharger are combined. In a controller, a target turbine flow rate for realizing a target compressor driving force is calculated; a target wastegate flow rate is calculated based on an exhaust gas flow rate and the target turbine flow rate; a target turbine-upstream pressure is calculated based on a target before/after-turbine pressure ratio for realizing the target compressor driving force and a turbine-downstream pressure; a target gate effective opening area is calculated based on the target wastegate flow rate, the target before/after-turbine pressure ratio, and the target turbine-upstream pressure; then, a gate valve control value is calculated.