An attachment (20; 320) mountable onto a surface of an optical component (10; 310), the surface of the optical component (10; 310) including an optical-component light passing unit (12a; 312a) configured to transmit light, the attachment (20; 320) being capable of shaping a light beam transmitted through the optical-component light passing unit (12a; 312a) and comprising: a light transmissive element (23; 323) configured to transmit light; an accommodating unit (21; 321) configured to accommodate the light transmissive element (23; 323); an attachment unit (22a; 22b) configured to attach the accommodating unit (21; 321) onto the optical component (10; 310) in such a manner that the light transmissive element (23; 323) faces the optical-component light passing unit (12a; 312a); and characterized by a sealant (80; 380; 480) configured to seal a light passing area where light passes being included in a space between a surface (12s; 312s) of the optical-component light passing unit (12a; 312a) facing the light transmissive element (23; 323) and a surface (23a; 323a) of the light transmissive element (23; 323) facing the optical-component light passing unit (12a; 312a) when the attachment (20; 320) is mounted on the optical component (10; 310), wherein the sealant (80; 380; 480) includes a contact portion (81; 381) configured to be compressed by and come in contact with the surface (12s; 312s) of the optical-component light passing unit (12a; 312a) and the surface (23a; 323a) of the light transmissive element (23; 323) and seal a circumference of the light passing area in the space when the attachment (20; 320) is mounted on the optical component (10; 310), a fixing portion (83; 383) spaced outwardly from the contact portion (81; 381) and configured to fix the contact portion (81; 381) to the accommodating unit (21; 321), and a connecting portion (40; 85; 340; 385; 385LU, 385LD, 385RU, 385RD) configured to connect the contact portion (81; 381) and the fixing portion (83; 383). a light transmissive element (23; 323) configured to transmit light; an accommodating unit (21; 321) configured to accommodate the light transmissive element (23; 323); an attachment unit (22a; 22b) configured to attach the accommodating unit (21; 321) onto the optical component (10; 310) in such a manner that the light transmissive element (23; 323) faces the optical-component light passing unit (12a; 312a); and characterized by a sealant (80; 380; 480) configured to seal a light passing area where light passes being included in a space between a surface (12s; 312s) of the optical-component light passing unit (12a; 312a) facing the light transmissive element (23; 323) and a surface (23a; 323a) of the light transmissive element (23; 323) facing the optical-component light passing unit (12a; 312a) when the attachment (20; 320) is mounted on the optical component (10; 310), wherein the sealant (80; 380; 480) includes a contact portion (81; 381) configured to be compressed by and come in contact with the surface (12s; 312s) of the optical-component light passing unit (12a; 312a) and the surface (23a; 323a) of the light transmissive element (23; 323) and seal a circumference of the light passing area in the space when the attachment (20; 320) is mounted on the optical component (10; 310), a fixing portion (83; 383) spaced outwardly from the contact portion (81; 381) and configured to fix the contact portion (81; 381) to the accommodating unit (21; 321), and a connecting portion (40; 85; 340; 385; 385LU, 385LD, 385RU, 385RD) configured to connect the contact portion (81; 381) and the fixing portion (83; 383). a contact portion (81; 381) configured to be compressed by and come in contact with the surface (12s; 312s) of the optical-component light passing unit (12a; 312a) and the surface (23a; 323a) of the light transmissive element (23; 323) and seal a circumference of the light passing area in the space when the attachment (20; 320) is mounted on the optical component (10; 310), a fixing portion (83; 383) spaced outwardly from the contact portion (81; 381) and configured to fix the contact portion (81; 381) to the accommodating unit (21; 321), and a connecting portion (40; 85; 340; 385; 385LU, 385LD, 385RU, 385RD) configured to connect the contact portion (81; 381) and the fixing portion (83; 383).