There is provided an exhaust gas purification apparatus for an internal combustion engine which can make the exhaust gas in which a reducing agent is diffused uniformly over the entirety flow into an NOx catalyst disposed on the downstream side.An exhaust gas purification apparatus for an internal combustion engine including an NOx catalyst disposed in an exhaust gas passage of an internal combustion engine, and a reducing agent injecting unit for injecting a reducing agent into the exhaust gas passage that is on an upstream side of the NOx catalyst, is characterized in that a narrowed portion in which the cross-sectional area of a flow passage is smaller than the cross-sectional area of the exhaust gas passage and that 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 that reducing agent diffusing means is provided on the narrowed portion.