A dragging device restricting one-way rotation of a spool of a fly reel. In a rotation-restricting device of a spool having a dragging device which allows the spool to be attached to and detached from a shaft secured to a housing and which restricts one-way rotation of the spool, a sleeve is fitted on the shaft, a flange extends in the radial direction from one end of the sleeve, a flange cover is secured to the housing in the spool-attached direction, the flange makes contact between the flange cover and the housing, and a disk plate makes contact with the flange, whereby pressure applied due to rotation of a drag-adjusting knob mounted on the housing is transferred to the disk plate to restrict one-way rotation of the spool. A waterproof structure on the friction surface of the disk plate prevents infiltration of water.