The invention provides an exhaust gas purification apparatus for an internal combustion engine which evenly diffuses a reducing agent over an entirety of the exhaust gas which flows into an NOx catalyst disposed on the downstream side of thereof. The exhaust gas purification apparatus includes an NOx catalyst disposed in an exhaust gas passage of an internal combustion engine, and a reducing agent injecting unit for injecting the reducing agent into the exhaust gas passage which is on an upstream side of the NOx catalyst. A narrowed portion, in which a cross-sectional area of a flow passage is smaller than a cross-sectional area of the exhaust gas passage and of the NOx catalyst, is provided on a downstream side of an injecting position by the reducing agent injecting unit and the upstream side of the NOx catalyst, and a reducing agent diffusing device is provided on the narrowed portion.