To provide an intake system for an internal combustion engine capable of relatively simply and properly reducing intake noise while keeping the intake efficiency of a multiple-line intake system.

Two intake ducts 15a and 15b are connected to an air cleaner 14 installed in a two-line intake system. The intake ducts 15a and 15b are integrated into one intake duct on their upstream side, and respectively connected to the air cleaner 14 by being branched in its intermediate part on the downstream side thereof. An auxiliary duct 17 having a part in its circumferential direction formed of a non-woven fabric 17a of a PET(polyethylene terephthalate) fiber is mounted in the upstream in the vicinity of the joining part of the intake ducts 15a and 15b.