To provide a union structure of a blow-by gas reducing device capable of preventing blocking of a passage due to freezing of moisture included in blow-by gas by a simple structure.

This blow-by gas reducing device returns blow-by gas leaking out to a crank camber of an engine to a surge tank (an intake air passage) 7 through a PCV hose 13. A union 12 for attaching one end of the PCV hose 13 is constituted as a double-cylinder structure of an inner cylinder 12A and an outer cylinder 12B. An air layer S formed between the inner cylinder 12A and the outer cylinder 12B of the union 12 functions as a heat insulating layer, and the blow-by gas flowing in the inner cylinder 12A is effectively insulated from the cryogenic outside air and the surge tank 7, so that the temperature in the inner cylinder 12A is kept above the freezing point. Freezing of moisture included in the blow-by gas flowing in the inner cylinder 12A is prevented, so that blocking of the union 12 due to freezing of the moisture can be prevented by a simple structure.