A helmet , comprising: a shell that includes a through hole extending from an outer side of the shell to an inner side of the shell ; and a grommet (10; 50) arranged on the shell where the through hole is located and forming a vent that draws outside air into the shell , wherein the grommet (10; 50) includes a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole , an operation member rotatably coupled to the base , and a plurality of shutters configured to open and close the opening when the operation member is rotated relative to the base , wherein the opening is a first opening , the operation member is a cover that covers the base and is rotated relative to the base , the plurality of shutters is located between the operation member and the base , and the operation member includes a second opening at a position corresponding to the first opening , characterized in that each of the plurality of shutters includes a guide protrusion projected toward the base , and a driven protrusion projected toward the operation member , the base includes a guide groove engaged with the guide protrusion , the operation member includes a cam groove engaged with the driven protrusion , and the operation member is configured to move the plurality of shutters in accordance with rotation of the operation member relative to the base . a shell that includes a through hole extending from an outer side of the shell to an inner side of the shell ; and a grommet (10; 50) arranged on the shell where the through hole is located and forming a vent that draws outside air into the shell , wherein the grommet (10; 50) includes a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole , an operation member rotatably coupled to the base , and a plurality of shutters configured to open and close the opening when the operation member is rotated relative to the base , a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole , an operation member rotatably coupled to the base , and a plurality of shutters configured to open and close the opening when the operation member is rotated relative to the base , wherein the opening is a first opening , the operation member is a cover that covers the base and is rotated relative to the base , the plurality of shutters is located between the operation member and the base , and the operation member includes a second opening at a position corresponding to the first opening , each of the plurality of shutters includes a guide protrusion projected toward the base , and a driven protrusion projected toward the operation member , a guide protrusion projected toward the base , and a driven protrusion projected toward the operation member , the base includes a guide groove engaged with the guide protrusion , the operation member includes a cam groove engaged with the driven protrusion , and the operation member is configured to move the plurality of shutters in accordance with rotation of the operation member relative to the base . <EOS>