A system, comprising: an upper rail; a slide rail to which the upper rail is slidably attached for a vehicle seat; and a light emitting device, comprising: a light emitter; and a holding portion locked to the upper rail and holding the light emitter inside the slide rail; characterized in that the light emitting device further comprises a light transmissive cover that covers the light emitter; the cover has a bottomed tubular shape, wherein a bottom portion covering the light emitter is formed in the cover at a position to face the light emitter in an irradiation direction of the light emitter, the irradiation direction being a direction in which the light emitter emits a light towards the bottom portion; the bottom portion of the cover has a shape that a protrusion amount protruding towards the irradiation direction increases as the bottom portion approaches a central axis of the light emitter on which the light of the light emitter becomes strongest; the bottom portion includes a plurality of outer surfaces, and the plurality of outer surfaces intersect with each other at a sharp angle; or the cover has a tubular shape, an opening is formed in the cover, and a direction in which the opening extends is an irradiation direction in which a light is emitted from the light emitter; the tubular shape of the cover is a rectangular tubular shape, and the cover includes a large rectangular tube portion and a small rectangular tube portion that is smaller than the large rectangular tube portion, along the irradiation direction of the light emitter. an upper rail; a slide rail to which the upper rail is slidably attached for a vehicle seat; and a light emitting device, comprising: a light emitter; and a holding portion locked to the upper rail and holding the light emitter inside the slide rail; characterized in that the light emitting device further comprises a light transmissive cover that covers the light emitter; the cover has a bottomed tubular shape, wherein a bottom portion covering the light emitter is formed in the cover at a position to face the light emitter in an irradiation direction of the light emitter, the irradiation direction being a direction in which the light emitter emits a light towards the bottom portion; the bottom portion of the cover has a shape that a protrusion amount protruding towards the irradiation direction increases as the bottom portion approaches a central axis of the light emitter on which the light of the light emitter becomes strongest; the bottom portion includes a plurality of outer surfaces, and the plurality of outer surfaces intersect with each other at a sharp angle; or the cover has a tubular shape, an opening is formed in the cover, and a direction in which the opening extends is an irradiation direction in which a light is emitted from the light emitter; the tubular shape of the cover is a rectangular tubular shape, and the cover includes a large rectangular tube portion and a small rectangular tube portion that is smaller than the large rectangular tube portion, along the irradiation direction of the light emitter. a light emitter; and a holding portion locked to the upper rail and holding the light emitter inside the slide rail; characterized in that the light emitting device further comprises a light transmissive cover that covers the light emitter; the cover has a bottomed tubular shape, wherein a bottom portion covering the light emitter is formed in the cover at a position to face the light emitter in an irradiation direction of the light emitter, the irradiation direction being a direction in which the light emitter emits a light towards the bottom portion; the bottom portion of the cover has a shape that a protrusion amount protruding towards the irradiation direction increases as the bottom portion approaches a central axis of the light emitter on which the light of the light emitter becomes strongest; the bottom portion includes a plurality of outer surfaces, and the plurality of outer surfaces intersect with each other at a sharp angle; or the cover has a tubular shape, an opening is formed in the cover, and a direction in which the opening extends is an irradiation direction in which a light is emitted from the light emitter; the tubular shape of the cover is a rectangular tubular shape, and the cover includes a large rectangular tube portion and a small rectangular tube portion that is smaller than the large rectangular tube portion, along the irradiation direction of the light emitter.