Disclosed is an EGR system of an internal combustion engine, in which a part of the exhaust gas is drawn out from an exhaust pipe and returned to an intake pipe through a control valve, and a reference pressure in the intake pipe corresponding to the amount of recirculated exhaust gas, predetermined by experiment (beforehand) in various combinations of engine intake air flow and speed, is memorized in a memorizing device. The reference pressure responding to the amount of sucked air being introduced into the engine and the engine rotating speed in the case of actual operation of the engine is read out from the memorizing device and, then, compared with the real pressure in the intake pipe in the case of actual operation of the engine. The control valve has a diaphragm chamber, into which positive or negative pressure is introduced, which pressure is controlled by a solenoid valve actuated by a signal from a control device so as to make the real intake pipe pressure coincide with the reference intake pipe pressure.