A sealing device with a rotation detecting element, suitable for sealing between an inner side of an engine and the atmosphere comprising: a stationary side sealing element which has an attachment ring fixed to the stationary side and a seal ring having a seal lip integrally formed with the attachment ring; a rotation side sealing element which is installed on the rotation side and has a seal flange extending to have a disc shape from an end part and slidably and hermetically contacting with the seal lip so as to seal fluid, and which has a cylindrical fitting part; and a magnetized rubber disc, wherein the sealing device further comprises a to-be-detected disc for rotation detection fitted to an end part at the opposite side of the seal flange in the rotation side sealing element, wherein the to-be-detected disc has a flange positioned at the inner peripheral side of the stationary side sealing element, extended to have a disc shape, and having the magnetized rubber disc, and a cylindrical fitting part; wherein an outer peripheral edge of the to-be-detected disc closely faces the inner peripheral surface of the stationary side sealing element in the radial direction; characterized in that the magnetic rubber disc is adhered on an outside surface of the flange of said to-be-detected disc, wherein an enlarged diameter cylindrical part, which is formed to have a larger diameter with an increment approximately corresponding to the thickness of the cylindrical fitting part of the to-be-detected disc is provided at the end part on the to-be-detected disc side of the cylindrical fitting part in the rotation side sealing element, and the cylindrical fitting part of the to-be-detected disc is pressed and fitted into the inner peripheral surface of the enlarged diameter cylindrical part. a stationary side sealing element which has an attachment ring fixed to the stationary side and a seal ring having a seal lip integrally formed with the attachment ring; a rotation side sealing element which is installed on the rotation side and has a seal flange extending to have a disc shape from an end part and slidably and hermetically contacting with the seal lip so as to seal fluid, and which has a cylindrical fitting part; and a magnetized rubber disc, wherein the sealing device further comprises a to-be-detected disc for rotation detection fitted to an end part at the opposite side of the seal flange in the rotation side sealing element, wherein the to-be-detected disc has a flange positioned at the inner peripheral side of the stationary side sealing element, extended to have a disc shape, and having the magnetized rubber disc, and a cylindrical fitting part; wherein an outer peripheral edge of the to-be-detected disc closely faces the inner peripheral surface of the stationary side sealing element in the radial direction; characterized in that the magnetic rubber disc is adhered on an outside surface of the flange of said to-be-detected disc, wherein an enlarged diameter cylindrical part, which is formed to have a larger diameter with an increment approximately corresponding to the thickness of the cylindrical fitting part of the to-be-detected disc is provided at the end part on the to-be-detected disc side of the cylindrical fitting part in the rotation side sealing element, and the cylindrical fitting part of the to-be-detected disc is pressed and fitted into the inner peripheral surface of the enlarged diameter cylindrical part.