Disclosed is an engaging structure of two cylindrical members. At least part of one of the two cylindrical members is coaxially fitted on the other cylindrical member. The engaging structure includes: a plurality of radial projections formed on the second cylindrical member; two annular flanges formed on the other cylindrical member to form a circumferential groove therebetween, the plurality of radial projections being fitted in the circumferential groove; and a plurality of cut-away portions, formed on one of the two annular flanges, through which the plurality of radial projections are respectively inserted into the circumferential groove. Each corresponding pair of the plurality of projections and the plurality of cut-away portions are formed to have a circumferential width different from those of the other corresponding pairs of the plurality of projections and the plurality of cut-away portions. At least one of corresponding pairs of the plurality of projections and the plurality of cut-away portions are formed to have a radial length different from those of the other corresponding pairs of the plurality of projections and the plurality of cut-away portions.