To provide an ammonia diluting device which can improve dissolving ability and treating ability of gaseous ammonia to be diluted.

The ammonia diluting device 1 which introduces and dissolves the gaseous ammonia into a dilution liquid stored in a dilution tank 2 of closed structure is provided with a blow line 8 for introducing a gas containing the gaseous ammonia, a liquid supply line 6 for supplying the dilution liquid, an ejector 4 for sucking and blending a liquid stream supplied from the liquid supply line 6 with a gas from the blow line 8 and ejecting the mixture into the dilution liquid stored in t the dilution tank 2 and an exhausting line 38 for exhausting the dilution liquid in which the gaseous ammonia is dissolved from the dilution tank 2.