A helmet (1) having a distribution chamber (15) provided in a chin cover portion (2a) formed directly below a window opening (3) of a cap body (2), an air intake hole (13) for introducing a travelling wind to the distribution chamber, and a plurality of jet holes (18) for blowing out the air of the distribution chamber (15) toward an inner surface of a shield plate (19) which is for closing the window opening (3), an air introducing member (30) attached to the upper edge of the window opening (3), and the air introducing member (30) being provided at a lower surface thereof with a plurality of guide grooves (33) formed deeper toward rearward, and an outlet (34) disposed at the rear end of the guide grooves (33) and opened toward the shield plate (19) to communicate with a ventilation passage (28,29) in the cap body. Rising air along the inner surface of the shield plate (19) is reliably formed to effectively prevent a fog of the inner surface of the shield plate (19).