A cooling device includes a drain passage, a collection passage and a bypass passage each connecting an upstream portion and a downstream portion of the drain passage, a branched portion branched into the collection passage and the bypass passage, and a confluence portion joined the collection passage and the bypass passage. A difference between flowing directions from the upstream portion into the branched portion and from the branched portion into the collection passage is smaller than a difference between flowing directions from the upstream portion into the branched portion and from the branched portion into the bypass passage. A difference between flowing directions from the bypass passage into the confluence portion and from the confluence portion into the downstream portion is smaller than a difference between flowing directions from the collection passage into the confluence portion and from the confluence portion into the downstream portion.