A device for purifying an exhaust gas of an internal combustion engine having a plurality of cylinders divided into first and second cylinder groups, and first and second exhaust passages connected to the first and the second cylinder groups, respectively, the device comprising: a NH3 synthesizing catalyst arranged in the first exhaust passage, the NH3 synthesizing catalyst synthesizing NH3 from at least of a part of Nox in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; first making-rich means for making the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH3 synthesizing catalyst rich to synthesize NH3; an interconnecting exhaust passage interconnecting the first passage downstream of the NH3 synthesizing catalyst and the second exhaust passage to contact NH3 synthesized in the NH3 synthesizing catalyst and Nox from the second exhaust passage with each other, to reduce the Nox by the NH3; and first making-lean means for making the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH3 synthesizing catalyst temporarily lean.