An inlet system includes an intake manifold and an EGR inlet projecting into and received in the manifold through an insertion portion of the manifold. A leading end of the inlet is inserted into the manifold through the insertion portion, projecting to reach a target position. An insertion portion opening diameter, which is an opening diameter of one side of the insertion portion, is larger than an insertion maximum occupying diameter that is a maximum distance between a point included in a cutting surface of the inlet on a reference plane perpendicular to an insertion portion axis and a point included in a vertical projection view of the leading end. A distance between the plane and a nearest point of the leading end to the plane in an axial direction is equal to an insertion portion opening depth, which is a length of the insertion portion in the axial direction.