An attaching structure of a rotation-detecting sensor comprises (a) a rotation-transferring medium, (b) a rotation-detecting sensor having (b1) a detection portion that detects a variation in magnetic field due to the rotation of the rotation-transferring medium to convert the variation into an electric signal and (b2) a main body having a plurality of side faces, (c) a flange that is provided at at least one side face of the main body of the rotation-detecting sensor, (d) an attaching member for the rotation-detecting sensor, the attaching member having an attaching hole into which the main body is inserted, concurrently with the flange being applied to the attaching member, and (e) a resin-molded body that (e1) covers by resin molding at least one part of the flange and a part of the attaching member to embed them in it and (e2) fixes the main body of the rotation-detecting sensor to the attaching member through the flange. The attaching hole of the attaching member also positions the rotation-detecting sensor. Even when the attaching hole is not filled with the resin-molded body because of the above-described positioning and the resin-molded body fails to achieve sufficient supporting force, the flange can securely fix the rotation-detecting sensor to the attaching member, because the flange is embedded in the resin-molded body and therefore can exercise the anchoring effect.