A lens unit attachment mechanism for attaching a lens unit so as to retain the lens unit in front of eyes of a helmet wearer, the lens unit attachment mechanism comprising: a lens unit configured by a frame member to support a pair of lenses and side frame members extending rearward of the lenses from left and right end portions of the frame member; angle adjustment members respectively coupled to the side frame members and adjusting angles of elevation and depression of the lens unit; a first member axially supporting such that each of the side frame members and each of the angle adjustment members are able to pivot with respect to each other; position adjustment members adjusting a horizontal direction position of the lens unit; a second member axially supporting such that each of the angle adjustment members and each of the position adjustment members are able to pivot with respect to each other; and position adjustment retention members respectively retaining helmet-side end portions of the position adjustment members so as to enable the position adjustment members to extend and retract in the horizontal direction, characterized in that the lens unit attachment mechanism further comprises a size adjustment member that is inserted between a first face oriented substantially parallel to the horizontal direction of one of the side frame members and a face opposing the first face, inserted between a second face oriented substantially parallel to the horizontal direction of one of the angle adjustment members and a face opposing the second face, or a combination thereof, such that the lens unit is adapted to be adjusted to a left-right direction width of a face of the helmet wearer. a lens unit configured by a frame member to support a pair of lenses and side frame members extending rearward of the lenses from left and right end portions of the frame member; angle adjustment members respectively coupled to the side frame members and adjusting angles of elevation and depression of the lens unit; a first member axially supporting such that each of the side frame members and each of the angle adjustment members are able to pivot with respect to each other; position adjustment members adjusting a horizontal direction position of the lens unit; a second member axially supporting such that each of the angle adjustment members and each of the position adjustment members are able to pivot with respect to each other; and position adjustment retention members respectively retaining helmet-side end portions of the position adjustment members so as to enable the position adjustment members to extend and retract in the horizontal direction, characterized in that the lens unit attachment mechanism further comprises a size adjustment member that is inserted between a first face oriented substantially parallel to the horizontal direction of one of the side frame members and a face opposing the first face, inserted between a second face oriented substantially parallel to the horizontal direction of one of the angle adjustment members and a face opposing the second face, or a combination thereof, such that the lens unit is adapted to be adjusted to a left-right direction width of a face of the helmet wearer.