In a breather device for an internal combustion engine for guiding blow-by gas generated in a crank chamber to an intake system, the breather device includes: a breather chamber formed adjacent to the crank chamber such that the breather chamber communicates with the crank chamber, the breather chamber being configured to separate oil from the blow-by gas that flows in the breather chamber from the crank chamber; a filter chamber formed adjacent to the crank chamber such that the filter chamber communicates with the breather chamber, the filter chamber accommodating a filter for separating oil from the blow-by gas that flows in the filter chamber from the breather chamber; and a breather passage that communicates the filter chamber and the intake system with each other.