An intake system for an internal combustion engine includes an intake passage connected with an intake port of the internal combustion engine; an EGR passage merged with the intake passage at a junction portion; a gas sensor attached to the intake passage and configured to detect a concentration of specific gas; and a control section configured to control the internal combustion engine on the basis of an output signal of the gas sensor. A diameter-enlarged portion having an inner diameter larger than an inner diameter of the junction portion is formed at a portion of the intake passage which is located on a downstream side of the junction portion. The gas sensor is located downstream from the diameter-enlarged portion.