Assembly structure for synthetic-resin component

An assembly structure includes a duct (11) including a protruding portion (31) provided in an outer circumference of the duct (11); a first component (13) including a first tubular fitting portion (34) configured to be fitted to a tip portion of the duct (11) by an insertion of the duct (11); and a second component (14) configured to be tip side of the second tubular fitting portion (35) and configured to be fitted to an outer circumferential surface of the first tubular fitting portion (34), a small diameter portion (37) located at a root side of the second tubular fitting portion (35) and configured to be fitted to the duct, and a step surface (38) located between the large diameter portion (30) and the small diameter portion (37). The step surface (38) faces a tip surface (41) of the first tubular fitting portion (34) to provide a concave portion (42) between the step surface (38) and the tip surface (41), and the protruding portion (31) is fitted to the concave portion (42), under a state where the duct (11) and the first and second components (13,14) have been attached to one another.