An alternator for vehicles, the alternator being provided with a rotary shaft, comprising slip rings fixed to a rotary shaft to be rotated with the rotary shaft, brushes being placed to be in slide contact with the slip rings a brush holder that accommodates the brushes to be held therein; connection terminals fixed to the brush holder, brush leads electrically connecting the brushes to the connection terminals respectively, and a slip ring cover that covers the slip rings in cooperation with the brush holder, wherein the brush holder comprises first chambers in which the bushes are fixedly accommodated second chambers in which the brush leads are accommodated, the second combers communicating with the first chambers, and first air holes formed to open outside of the housing and formed as inlet/exhaust passages communicating with the second chambers.