Conforming with the differing conditions in which the left and right headlight lamps are placed, an anti-fogging structure for headlight lamps is proposed wherein variation in the anti-fogging performance on the left and right does not occur by providing differing structures for the breathing mechanisms in the left and right headlight lamps. In an anti-fogging mechanism for headlight lamps 5L and 5R provided on the left and right of the radiator fan at the front of the body of a vehicle, different structures are provided for the breathing holes that enable circulation of air into and out of the lamp housings 51L and 51R of the left and right headlight lamps 5L and 5R.