A device for opening-closing jaw guard of a helmet is disclosed. The device has a hinge means 400 comprising fixed plates 410 which are fixed on both the left and right sides of the helmet body 1 and each of which has a long hole 411, operating plates 420 which are fixed on the inside of the left and right extending portions 4a of the jaw guard 4 and each of which has an arc-shaped internal gear 421, a planetary gear 430 which is engaged with the internal gear 421 of the operating plate 420, and has a protuberance 431 formed on either side, and a cutaway portion 432 formed on the outer circumference; and a pivoting lever 440 which has a shaft protuberance 441 of front end inserted into the cutaway portion 432 of the planetary gear 430 and a penetration hole 442 of rear end inserted into the main boss 423 of the operating plate 420.