A duct 20 includes: wall portions 24, each of which defines a corresponding one face of an air flow path 22 leading air from an air inlet 20a to an air outlet 20b; and groove portions 26, which are provided on at least two of the wall portions 24, 24, respectively, and extend so as to cross an air flow direction of the air flow path 22, and the groove portions 26 are independent from each other for every wall portion 24, and have a concave shape so as to thin the wall portions 24.