To allow the exhaust gas of the diesel engine not to cause pollution and not to become a factor in an atmospheric pollution or the global warming with CO2.

The gas exhausted from Diesel engine 1 is washed in a water tank 2 splashed in mist form, and the washed exhaust gas passes through a filter A3 and an activated charcoal filter 4 to be purified in a chamber of a generator device 5 for negative ions and photocatalyst consisting of a negative ion generator, light emitter, and photocatalyst. The exhaust gas is further purified by an air cleaner 6 and exhausted.