An exhaust purification system that can, in a wider temperature range, reduce and remove nitrogen oxide in exhaust gas by a reducing catalyst with the use of hydrogen as a reducing agent. The exhaust purification system has an electronic control device (41). The electronic control device (41) controls an EGR valve (4) and an EGR pipe (5) so that the concentration of oxygen in exhaust gas, which concentration is obtained from data map based on conditions of the engine (6), becomes less than a predetermined value, and the electronic control device (41) also controls a microreactor (19) so that hydrogen is added to the exhaust gas when the concentration of oxygen in the exhaust gas is less than the predetermined value.