A basic opening (A0) of an EGR control valve (22) is set, based on a current engine operation state. A differential pressure (ΔP1) across the EGR control valve (22) is calculated, based on an actual exhaust system temperature (T1) sensed by an exhaust temperature sensor (33). A reference differential pressure (ΔP0) is calculated, which is a differential pressure across the EGR control valve (22) in a steady state corresponding to the current engine operation state. A reference pulsation amplitude (D) is calculated, which is an amplitude of pulsation of the reference differential pressure (ΔP0). The basic opening (A0) is corrected, based on the differential pressure (ΔP1), the reference differential pressure (ΔP0), and the reference pulsation amplitude (D).