An internal combustion engine intake duct device is provided with an intake duct body and an air flow directing structure. The intake duct body has an inlet, a left outlet and a right outlet with an intake air passage formed in the intake duct body for supplying intake air from the inlet to left and right outlets. The inlet is disposed on an opposite side of the intake duct body from the left and right outlets and laterally between the left and right outlets in a top plan view. The air flow directing structure is arranged in the intake duct body to direct a flow of intake air entering from the inlet toward the left and right outlets. The air flow directing structure includes a first sound reflecting part serving to block sound from the left intake pipe and a second sound reflecting part serving to block sound from the right intake pipe.