A device for opening-closing sun visor of a helmet is disclosed. The device comprises a helmet body 10 provided in front with an opening portion 11 for the face and having a pair of hinge shafts 12 protruded on both the left and right sides, a sun visor 20 having a visor portion 21 placed on the side of the opening portion 11 of the helmet body 10, extending portions 22 extended to the left and right sides of the visor portion 21, and hinge slots 23 formed at the ends of the extending portions 22; and an opening-closing mechanism 30 having a slider 31 connected to the top center of the visor portion 21 of the sun visor 20, a guide rail 32 which is attached to the ceiling of the helmet body 10, and a guide holder 35 which is attached lengthwise to the top center of the helmet body.