An embodiment provides a damper a damper including: a housing; a rotor; and a sealing member. The housing includes outer and inner cylinder portions to define a filling space portion therebetween for filling with a viscose fluid. The rotor includes a head portion and a cylinder portion extended downwardly therefrom so as to be inserted into the filling space portion. The sealing member seals the filling space portion. The rotor further includes axially-extending first and second groove portions provided on outer and inner peripheral surfaces of the cylinder portion near the bottom portion thereof. At least one of the first groove portions includes a communicating portion which communicates the inside and the outside of the rotor, and the second groove portion is apart from the communicating portion in a circumferential direction.