To provide an exhaust gas post-treatment system that reduces a decrease in an engine output and an increase in a fuel consumption which take place when a gasoline particle filter that removes particulates in an exhaust gas inside a discharge line of a direct injection gasoline engine is mounted.

The exhaust gas post-treatment system includes a discharge line through which the exhaust gas discharged from an engine passes, a gasoline particle filter provided in the discharge line, a by-pass line provided in the discharge line so as to make the exhaust gas by-pass the gasoline particle filter, a main valve that blocks the exhaust gas that is made to flow into the gasoline particle filter, a by-pass valve that blocks the exhaust gas passing through the by-pass line, and a control section that detects operating conditions of the engine, controls the exhaust gas so as to pass through the gasoline particle filter, or controls the exhaust gas so as to pass through the by-pass line without passing through the gasoline particle filter.