A manual variable optical attenuator (10) includes a base (2), a cover (3), an optical module (4), a shifting device (5), and an optical fiber retainer (6). The optical module comprises a mirror (41), a frame (42), a dual optical fiber collimator (43), a stopper (44), a graded transmittance filter (45), and a filter carrier (46). The shifting device comprises a screw pole (51), a first holder (52), and a second holder (53). The screw pole comprises a terminal portion (511) rotatably received in the first holder, a first annular groove (512) accommodating the second holder, a second annular groove (513) receiving a gasket (514), an adjusting slot (515), and a screw thread (516). T he gasket protrudes from a circumferential surface of the screw pole and seals a gap between the screw pole and a sidewall (36) of the cover. External harmful contamination cannot penetrate into the attenuator.