An oxidation catalyst, an ozone injection nozzle, a urea-based fluid injection nozzle, and a selective reduction catalyst are arranged in an exhaust pipe of an engine in this order towards its outlet. An exhaust gas purifier includes an air flow sensor that detects an air intake amount of the engine, a NOx sensor that detects a NOx concentration in the exhaust pipe, and first and second temperature sensors that detect inlet temperatures of the catalysts. An ozone generation device to which the ozone injection nozzle is connected includes a compressor, a dryer that dries compressed air, a flow rate sensor that detects a flow rate of this air, an ozone generator that converts this air into ozone, and an ozone concentration sensor that detects a concentration of the ozone. A controller controls the compressor and the ozone generator based on respective detection outputs of the respective sensors.