To successively execute favorable reduction and purification of NOx even under conditions of drop in urea water to less than the necessary minimum amount (less than a set lower limit value of a level sensor) and of abnormality in concentrations.

A reducing agent supply device for supplying reducing agent to a selection reduction type catalyst 10 provided at some midpoint in an exhaust pipe 9, is equipped with a urea water tank 14 for storing urea water 11 as reducing agent for regular use, a fuel tank 18 for storing light oil 12 as an engine fuel and an injection device 19 for selectively leading the urea water 11 or light oil 12 from either of the urea water tank 14 or fuel tank 18 through the medium of a three-way switch valve 17 (a flow path switching means) and injecting the urea water 11 or light oil 12 to the inlet side of the selective reduction catalyst 10.

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