A seal device for a bicycle steering bearing assembly includes a seal ring having a tubular insertion and a flange extends radially horizontally from one end of the insertion. A tapered mask has a first open end which is engaged with a groove in an outer periphery of the insertion and a second open end of the mask receives the steering bearing assembly therein. A locking nut is securely mounted to the steerer tube and has another groove defined in an inner periphery of one end thereof. The other end of the insertion is engaged with the groove of the locking nut.