A resin member fitting structure including a first resin member, which is a rectangular hollow tube, has at least one inner wall surface. A second resin member, which is a hollow or solid rectangular tube, has at least one outer wall surface fitted to the at least one inner wall surface of the first resin member. The inner wall surface of the first resin member and the outer wall surface of the second resin member come into contact with each other and define at least one set of fitting surfaces. The second resin member has a hygroscopic expansion coefficient that is smaller than that of the first resin member. The at least one fitting surface includes a plurality of recess-projection engagement structures spaced apart from each other in a circumferential direction that intersects a longitudinal direction of the first and second resin members.